[MOV] content/*: move resource files into their related page's directory

Since odoo/documentation#903, the guideline for the location of new
resource (images, downloadable files, RST includes...) files is to place
those inside the directory of the RST page that references them.

For example, if `doc1.rst` has a reference to `image.png` and to
`download.zip`, the file structure should look like this:

├── parent_doc/
│     └── doc1/
│     │     └── image.png
│     │     └── download.zip
│     └── doc1.rst
│     └── doc2.rst
├── parent_doc.rst

Before this commit, most of the resource files were still located inside
'media' directories holding all the resource files referenced by RST
pages located at the same level as these directories. In the example
above, a single 'media' directory would hold all the resource files
referenced by both `doc1.rst` and `doc2.rst`. Doing so prevented us from
figuring out easily which resource file was referenced by which RST page
and, thus, lead to unused resource files piling up in the repository. It
also made it more complicated to define codeowners regex rules because a
team could not simply be assigned to `/some_page.*` but needed to be
assigned to both `/some_page\.rst` and to the location of 'media'.

In order to help new content writers figure out the guideline when
taking examples from other RST pages, this commit retroactively applies
the guideline to existing resource files and 'media' directories. The
left-over resource files that are not referenced by any RST page are
removed.

task-2497965

Part-of: odoo/documentation#2068

content/applications/sales/point_of_sale/overview/getting_started.rst

@@ -20,7 +20,7 @@ To make products available for sale, go to :menuselection:`Point of
 Sale --> Products --> Products` and open a product. In the *Sales*
 tab, enable *Available in Point of Sale*.
 
-.. image:: media/getting_started_01.png
+.. image:: getting_started/getting_started_01.png
     :align: center
     :alt: Product form. Making the product available in PoS.
 
@@ -35,7 +35,7 @@ To add a payment method, you first need to create it. Go to
 and click on create. Then, you can name your payment method and set it
 up.
 
-.. image:: media/getting_started_02.png
+.. image:: getting_started/getting_started_02.png
     :align: center
     :alt: Creating a new payment method for a Point of Sale.
 
@@ -47,7 +47,7 @@ go to :menuselection:`Point of Sale --> Configuration --> Point of
 Sale` and open the PoS in which you want to include the payment method. Then, add the
 payment method.
 
-.. image:: media/getting_started_03.png
+.. image:: getting_started/getting_started_03.png
     :align: center
     :alt: Making the payment method available in a point of sale.
 
@@ -63,13 +63,13 @@ Your first order
 
 Open a new session from the dashboard by clicking on *New Session*.
 
-.. image:: media/getting_started_04.png
+.. image:: getting_started/getting_started_04.png
     :align: center
     :alt: Launching a new session.
 
 After the loading screen, you arrive on the PoS interface.
 
-.. image:: media/getting_started_05.png
+.. image:: getting_started/getting_started_05.png
     :align: center
     :alt: Point of sale dashboard.
 
@@ -90,14 +90,14 @@ of accepting returns and refunds easy for you.
 To do that, from the PoS interface, select the product and quantity (with the +/- button) that the 
 customer wants to return. For multiple products, repeat the process individually.
 
-.. image:: media/getting_started_06.png
+.. image:: getting_started/getting_started_06.png
     :align: center
     :alt: Refunding a product with the point of sale app.
 
 When on the payment interface, the total is negative. To end the refund,
 process the payment and validate it.
 
-.. image:: media/getting_started_07.png
+.. image:: getting_started/getting_started_07.png
     :align: center
     :alt: Giving back money to customer in Odoo Point of Sale.
 
@@ -107,7 +107,7 @@ Close the PoS session
 To close your session at the end of the day, click on the close button on the upper right corner 
 of your screen and confirm. Now, close the session on the dashboard view.
 
-.. image:: media/getting_started_08.png
+.. image:: getting_started/getting_started_08.png
     :align: center
     :alt: How to close a point of sale session.
 
@@ -118,7 +118,7 @@ Once a session is closed, you can see a summary of all transactions per payment
 Then, click on a line to see all orders that were paid during your PoS session. 
 If everything is correct, validate the session and post the closing entries.
 
-.. image:: media/getting_started_09.png
+.. image:: getting_started/getting_started_09.png
     :align: center
     :alt: Point of sale sales report before closing the session.
 
@@ -138,12 +138,12 @@ To access your statistics, go to :menuselection:`Point of Sales -->
 Reporting --> Orders`. There, you can see various statistics in graph
 or pivot form.
 
-.. image:: media/getting_started_10.png
+.. image:: getting_started/getting_started_10.png
     :align: center
     :alt: Orders analysis with pivot view of the point of sale reporting feature.
 
 You can also access them through the dashboard.
 
-.. image:: media/getting_started_11.png
+.. image:: getting_started/getting_started_11.png
     :align: center
     :alt: Accessing orders analysis via the point of sale dashboard. Reporting feature on the PoS form.

content/applications/sales/point_of_sale/overview/register.rst

@@ -12,18 +12,18 @@ Create a customer
 
 From your session interface, use the customer button.
 
-.. image:: media/register01.png
+.. image:: register/register01.png
     :align: center
 
 Create a new one by using this button.
 
-.. image:: media/register02.png
+.. image:: register/register02.png
     :align: center
 
 You will be invited to fill out the customer form with their
 information.
 
-.. image:: media/register03.png
+.. image:: register/register03.png
     :align: center
 
 Use the save button when you are done. You can then select that customer