[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/payment/six.rst

@@ -20,7 +20,7 @@ Configure the Payment Method
 First, make sure that the POS Six module is installed. For this, go to *Apps*,
 remove the "Apps" filter and search for "POS Six".
 
-.. image:: media/six_01.png
+.. image:: six/six_01.png
     :align: center
 
 Back in :menuselection:`Point of Sale --> Configuration --> Payment Methods`, create a new payment
@@ -38,14 +38,14 @@ method using a payment terminal. Verify that the amount in the tendered
 column is the one that has to be sent to the payment terminal and click
 on *Send*. If you want to cancel the payment request, click on cancel.
 
-.. image:: media/six_06.png
+.. image:: six/six_06.png
     :align: center
 
 When the payment is done, the status will change to *Payment
 Successful*. You can always reverse the last transaction by clicking on
 *Reverse*.
 
-.. image:: media/six_07.png
+.. image:: six/six_07.png
     :align: center
 
 If there is any issue with the payment terminal, you can still force the