[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/websites/ecommerce/maximizing_revenue/cross_selling.rst

@@ -14,12 +14,12 @@ Accessory products when checking out
 Accessories (e.g. for computers: mouse, keyboard) show up when the customer
 reviews the cart before paying.
 
-.. image:: ./media/cross_sell_accessory_web.png
+.. image:: cross_selling/cross_sell_accessory_web.png
    :align: center
 
 Select accessories in the *Sales* tab of the product detail page.
 
-.. image:: ./media/cross_sell_accessory.png
+.. image:: cross_selling/cross_sell_accessory.png
    :align: center
 
 .. note::
@@ -34,7 +34,7 @@ Optional products are directly related to the item added to cart (e.g. for compu
 warranty, OS software, extra components). Whenever the main product is added to cart,
 such a new screen pops up as an extra step.
 
-.. image:: ./media/cross_sell_optional_web.png
+.. image:: cross_selling/cross_sell_optional_web.png
    :align: center
 
 To publish optional products:
@@ -42,12 +42,12 @@ To publish optional products:
 * Install *eCommerce Optional Products* addon in *Apps* menu. Remove the default filter to
   search on addons as well, otherwise only main apps show up.
 
-.. image:: ./media/cross_sell_optional_install.png
+.. image:: cross_selling/cross_sell_optional_install.png
    :align: center
 
 * Select optional items from the *Sales* tab of the product detail form.
 
-.. image:: ./media/cross_sell_optional.png
+.. image:: cross_selling/cross_sell_optional.png
    :align: center
 
 .. note::