[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/productivity/discuss/overview/get_started.rst

@@ -14,7 +14,7 @@ Choose your notifications preference
 
 Access your *Preferences* and choose how you would like your notifications to be handled.
 
-.. image:: media/change_preferences.png
+.. image:: get_started/change_preferences.png
    :align: center
    :alt: View of the preferences page for Odoo Discuss
 
@@ -23,14 +23,14 @@ Access your *Preferences* and choose how you would like your notifications to be
   they are shown in the *Inbox*.
 | Messages can then be *Marked as Todo*, *Replied*, or *Marked as Read*.
 
-.. image:: media/inbox_actions.png
+.. image:: get_started/inbox_actions.png
    :align: center
    :alt: View of an inbox message and its action options in Odoo Discuss
 
 The messages tagged as *Mark as Todo* are also shown in *Starred*, while the ones *Marked as Read*
 are moved to *History*.
 
-.. image:: media/starred_messages.png
+.. image:: get_started/starred_messages.png
    :align: center
    :alt: View of messages marked as todo in Odoo Discuss
 
@@ -41,7 +41,7 @@ The first time you log in to your account, OdooBot sends you a message asking fo
 receive desktop notifications to chats. If accepted, you receive push notifications to the messages
 sent to you despite of where you are in Odoo.
 
-.. image:: media/odoobot_push.png
+.. image:: get_started/odoobot_push.png
    :align: center
    :alt: View of the messages under the messaging menu emphasizing the request for push
          notifications for Odoo Discuss
@@ -52,7 +52,7 @@ sent to you despite of where you are in Odoo.
 To start a chat, click on *New Message* on the *Messaging Menu*, or go to *Discuss* and send a
 *Direct Message*.
 
-.. image:: media/channels_direct_messages.png
+.. image:: get_started/channels_direct_messages.png
    :align: center
    :height: 400
    :alt: View of discuss’s panel emphasizing the titles channels and direct messages in Odoo Discuss
@@ -67,7 +67,7 @@ Mentions in the chat and on the Chatter
 | A notification is sent to the user mentioned either to his *Inbox* or through email, depending
   on his settings.
 
-.. image:: media/chat_windows.png
+.. image:: get_started/chat_windows.png
    :align: center
    :alt: View of a couple of chat window messages for Odoo Discuss
 
@@ -88,7 +88,7 @@ sidebar and on the *Messaging menu*.
 - White = offline
 - Airplane = out of the office
 
-.. image:: media/status.png
+.. image:: get_started/status.png
    :align: center
    :height: 300
    :alt: View of the contacts’ status for Odoo Discuss