[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/services/helpdesk/overview/reports.rst

@@ -18,7 +18,7 @@ Some examples of the reports Odoo Helpdesk can generate include:
 | Apply *Time Ranges* if you would like to make comparisons to a *Previous Period* or a *Previous
   Year*.
 
-.. image:: media/report_team_ticket_type.png
+.. image:: reports/report_team_ticket_type.png
    :align: center
    :alt: View of a helpdesk ticket analysis by team and ticket type in Odoo Helpdesk
 
@@ -27,7 +27,7 @@ Some examples of the reports Odoo Helpdesk can generate include:
 Get an overview of how many requests each team is closing per day in order to measure their
 performance. Identify productivity levels to understand how many requests they are able to handle.
 
-.. image:: media/report_close_date_team.png
+.. image:: reports/report_close_date_team.png
    :align: center
    :alt: View of a helpdesk ticket analysis by team and close date in Odoo Helpdesk
 
@@ -39,7 +39,7 @@ performance. Identify productivity levels to understand how many requests they a
 Check if your expectations are met by *measuring* the *Time to close (hours)*. Your
 customers not only expect fast responses but they also want their issues to be handled quickly.
 
-.. image:: media/report_time_to_close.png
+.. image:: reports/report_time_to_close.png
    :align: center
    :alt: View of helpdesk ticket analysis of the hours to close by ticket type and team in
          Odoo Helpdesk
@@ -50,7 +50,7 @@ Save filters
 Save the filters you use the most and avoid having to reconstruct them every time they are
 needed. To do so, set the groups, filters, and measures needed. Then, go to *Favorites*.
 
-.. image:: media/favorites.png
+.. image:: reports/favorites.png
    :align: center
    :alt: View of helpdesk ticket analysis emphasizing the option to add a filter as a favorite one
          in Odoo Helpdesk