[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/finance/accounting/others/analytic/usage.rst

@@ -13,7 +13,7 @@ The analytic accounting can be used for several purposes:
 To manage analytic accounting, you have to activate it in
 :menuselection:`Configuration --> Settings`:
 
-.. image:: media/usage01.png
+.. image:: usage/usage01.png
    :align: center
 
 To illustrate analytic accounts clearly, you will follow three use
@@ -88,7 +88,7 @@ The analytic representation by department enables you to investigate the
 costs allocated to each department in the company. The analytic chart of
 accounts shows the distribution of the company's costs using the example above:
 
-.. image:: media/usage02.png
+.. image:: usage/usage02.png
    :align: center
 
 In this example of a hierarchical structure in Odoo, you can analyse not
@@ -97,7 +97,7 @@ production. A report that relates both general accounts and analytic
 accounts enables you to get a breakdown of costs within a given
 department.
 
-.. image:: media/usage03.png
+.. image:: usage/usage03.png
    :align: center
 
 The examples above are based on a breakdown of the costs of the company.