[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#2064

content/applications/sales/subscriptions/sales_flow/closing.rst

@@ -35,7 +35,7 @@ create a new *Subscription template* or modify an existing one. When editing you
 the Invoicing tab, you have the possibility to activate the option *Closable by customer*.
 Once enabled, this option gives your customers the right to close their own subscriptions.
 
-.. image:: media/configuration-to-close-your-subscriptions.png
+.. image:: closing/configuration-to-close-your-subscriptions.png
   :align: center
   :alt: Configuration to close your subscription with Odoo Subscriptions
 
@@ -55,7 +55,7 @@ Once confirmed, a quotation becomes a sales order and a new subscription is auto
 Therefore, this subscription has the status *In progress*. From there, you have the possibility to
 close the subscription.
 
-.. image:: media/close-your-subscriptions-as-an-administrator.png
+.. image:: closing/close-your-subscriptions-as-an-administrator.png
   :align: center
   :alt: Close your subscription from an administration point of view with Odoo Subscriptions
 
@@ -64,7 +64,7 @@ expensive", "Subscription does not meet my requirements", "Subscription reached
 Immediately after confirming your close reason, you can observe that the status of the subscription
 is now *Closed* and that the close reason is mentioned on the subscription.
 
-.. image:: media/use-of-close-reasons.png
+.. image:: closing/use-of-close-reasons.png
   :align: center
   :alt: What happens when you close your subscription with Odoo Subscriptions?
 
@@ -76,14 +76,14 @@ the possibility to visualize what your customers see when managing their subscri
 *Customer preview* button. In this example, the customer has the choice to close his/her subscription
 whenever he/she wants to, due to the *Close Subscription* button.
 
-.. image:: media/close-your-subscriptions-as-a-customer.png
+.. image:: closing/close-your-subscriptions-as-a-customer.png
   :align: center
   :alt: Close your subscription from a customer point of view with Odoo Subscriptions
 
 By using this button, the customer can specify the reason for cancelling his/her subscription and
 he/she can even leave a message.
 
-.. image:: media/use-of-close-reasons-as-a-customer.png
+.. image:: closing/use-of-close-reasons-as-a-customer.png
   :align: center
   :alt: What happens when customers close their subscription with Odoo Subscriptions?
 
@@ -92,7 +92,7 @@ informed of this modification. Indeed, the status of the subscription becomes *C
 appears in the chatter with the new stage, the end date, the close reason, and the closing text
 added by the customer.
 
-.. image:: media/chatter-history-when-a-customer-closes-a-subscription.png
+.. image:: closing/chatter-history-when-a-customer-closes-a-subscription.png
   :align: center
   :alt: What happens when customers close their subscription in Odoo Subscriptions?