[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/finance/accounting/receivables/customer_payments/check.rst

@@ -59,7 +59,7 @@ on **Register Payment**. Fill in the information about the payment:
 
 -  Memo: write the Check number
 
-.. image:: ./media/check02.png
+.. image:: check/check02.png
   :align: center
 
 This operation will produce the following journal entry:
@@ -115,7 +115,7 @@ on **Register Payment**. Fill in the information about the payment:
 
 -  Memo: write the check number
 
-.. image:: ./media/check03.png
+.. image:: check/check03.png
   :align: center
 
 The invoice is marked as paid as soon as you record the check.
@@ -147,5 +147,5 @@ report **Bank Reconciliation Report** to verify which checks have been
 received or paid by the bank. (this report is available from the **More**
 option from the Accounting dashboard on the related bank account).
 
-.. image:: ./media/check01.png
+.. image:: check/check01.png
   :align: center