[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#2066

content/applications/services/helpdesk/overview/getting_started.rst

@@ -13,7 +13,7 @@ Set up teams
 | Setting up multiple teams allows you to group tickets by your channels (example: BE/US), or by
   your support services' types (example: IT, accounting, admin, etc.).
 
-.. image:: media/helpdesk_teams_view.png
+.. image:: getting_started/helpdesk_teams_view.png
    :align: center
    :alt: View of the helpdesk teams page in Odoo Helpdesk
 
@@ -30,7 +30,7 @@ person:
 - *Balanced*: tickets are assigned to the person with the least amount of tickets so that everyone
   fairly gets the same amount. Thereby, you ensure that all tickets get to be taken care of.
 
-.. image:: media/productivity_visibility.png
+.. image:: getting_started/productivity_visibility.png
    :align: center
    :alt: View of a helpdesk team settings page emphasizing the productivity and visibility features
          in Odoo Helpdesk
@@ -48,7 +48,7 @@ Set up stages and share it among teams
 To set up stages, go to :menuselection:`Helpdesk --> Configuration --> Stages`. Then, create and/or
 edit stages as you need and set specific teams to use certain stages under *Team*.
 
-.. image:: media/stages_teams.png
+.. image:: getting_started/stages_teams.png
    :align: center
    :alt: View of a stage’s setting page emphasizing the option to add teams in Odoo Helpdesk
 
@@ -56,7 +56,7 @@ Stages can be shared between one or multiple teams, allowing you to adapt the pi
 individual needs. They also apply a visibility and access rule, as other teams are not able to see
 or use the stage.
 
-.. image:: media/helpdesk_kanbanview.png
+.. image:: getting_started/helpdesk_kanbanview.png
    :align: center
    :alt: View of a team’s kanban view in Odoo Helpdesk