[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/developer/howtos/rdtraining/03_newapp.rst

@@ -18,7 +18,7 @@ to answer the specific business case.
 
 Here is an overview of the main list view containing some advertisements:
 
-.. image:: 03_newapp/media/overview_list_view_01.png
+.. image:: 03_newapp/overview_list_view_01.png
    :align: center
    :alt: List view 01
 
@@ -26,14 +26,14 @@ The top area of the form view summarizes important information for the property,
 the property type, the postcode and so on. The first tab contains information describing the
 property: bedrooms, living area, garage, garden...
 
-.. image:: 03_newapp/media/overview_form_view_01.png
+.. image:: 03_newapp/overview_form_view_01.png
    :align: center
    :alt: Form view 01
 
 The second tab lists the offers for the property. We can see here that potential buyers can make
 offers above or below the expected selling price. It is up to the seller to accept an offer.
 
-.. image:: 03_newapp/media/overview_form_view_02.png
+.. image:: 03_newapp/overview_form_view_02.png
    :align: center
    :alt: Form view 02
 
@@ -52,7 +52,7 @@ Prepare the addon directory
    **Goal**: the goal of this section is to have Odoo recognize our new module, which will
    be an empty shell for now. It will be listed in the Apps:
 
-   .. image:: 03_newapp/media/app_in_list.png
+   .. image:: 03_newapp/app_in_list.png
       :align: center
       :alt: The new module appears in the list