[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/services/timesheets/overview/time_off.rst

@@ -9,14 +9,14 @@ and questions after hours that have not been timesheeted by the employee.
 Activate the :ref:`developer mode <developer-mode>`, go to *Timesheets*, and change the *Project*
 and *Task* set by default, if you like.
 
-.. image:: media/record_time_off.png
+.. image:: time_off/record_time_off.png
    :align: center
    :alt: View of Timesheets setting enabling the feature record time off in Odoo Timesheets
 
 Go to :menuselection:`Time Off --> Configuration --> Time Off Types`. Select or create the
 needed type, and decide if you would like the requests to be validated or not.
 
-.. image:: media/time_off_types.png
+.. image:: time_off/time_off_types.png
    :align: center
    :alt: View of a time off types form emphasizing the time off requests and timesheets section in
          Odoo Time Off
@@ -26,7 +26,7 @@ needed type, and decide if you would like the requests to be validated or not.
   respective project and task.
 | On the example below, the user requested *Paid Time off* from July 13th to 15th.
 
-.. image:: media/time_off_request.png
+.. image:: time_off/time_off_request.png
    :align: center
    :alt: View of the time off request form in Odoo Time Off
 
@@ -34,13 +34,13 @@ Considering that validation is not required, the requested time off is automatic
 *Timesheets*. If validation is necessary, the time is automatically allocated after the responsible
 person for validating does it so.
 
-.. image:: media/timesheets.png
+.. image:: time_off/timesheets.png
    :align: center
    :alt: Video of timesheets emphasizing the requested time off from the employee in Odoo Timesheets
 
 Click on the magnifying glass, hovering over the concerned cell, to access all the aggregated data
 on that cell (day), and see details regarding the project/task.
 
-.. image:: media/timesheet_description.png
+.. image:: time_off/timesheet_description.png
    :align: center
    :alt: View of the details of a project/task in Odoo Timeheets