[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#2066

content/applications/sales/subscriptions/configuration/subscription_templates.rst

@@ -35,7 +35,7 @@ Go to :menuselection:`Subscriptions --> Configuration --> Subscription templates
 suggests you two types of subscription (MON - Monthly subscription *vs* YEA - Yearly subscription).
 You can also create your own ones.
 
-.. image:: media/default-subscription-templates.png
+.. image:: subscription_templates/default-subscription-templates.png
   :align: center
   :alt: Default subscription templates on Odoo Subscriptions
 
@@ -54,7 +54,7 @@ Among the payment options, an additional field called *Invoice email* appears wh
 *Send*, *Send & try to charge* or *Send after successful payment*. This field allows you to add an
 invoice email template to your subscription templates.
 
-.. image:: media/creation-of-subscription-templates.png
+.. image:: subscription_templates/creation-of-subscription-templates.png
   :align: center
   :alt: Create your own subscription templates on Odoo Subscriptions
 
@@ -67,7 +67,7 @@ subscriptions or not. If enabled, you can set an *Automatic closing* limit and s
    essential to set out important contractual points between the customers and the sellers (payment,
    refund policy, cancellation, complaints, etc.).
 
-   .. image:: media/terms-and-conditions-on-subscription-templates.png
+   .. image:: subscription_templates/terms-and-conditions-on-subscription-templates.png
      :align: center
      :alt: Terms & conditions on Odoo Subscriptions
 
@@ -77,7 +77,7 @@ subscriptions or not. If enabled, you can set an *Automatic closing* limit and s
    summarizes all the records corresponding to these filters and you are able to manage them in one
    click.
 
-   .. image:: media/health-check-on-subscription-templates.png
+   .. image:: subscription_templates/health-check-on-subscription-templates.png
      :align: center
      :alt: Health check on Odoo Subscriptions