[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/sales/crm/track_leads/prospect_visits.rst

@@ -14,7 +14,7 @@ Configuration
 To use this feature, install the free module *Lead Scoring* under your
 *Apps* page (only available in Odoo Enterprise).
 
-.. image:: media/prospect_visits01.png
+.. image:: prospect_visits/prospect_visits01.png
    :align: center
 
 Track a webpage
@@ -23,12 +23,12 @@ Track a webpage
 Go to any static page you want to track on your website and under the
 *Promote* tab you will find *Optimize SEO*
 
-.. image:: media/prospect_visits02.png
+.. image:: prospect_visits/prospect_visits02.png
    :align: center
 
 There you will see a *Track Page* checkbox to track this page.
 
-.. image:: media/prospect_visits03.png
+.. image:: prospect_visits/prospect_visits03.png
    :align: center
 
 See visited pages in your leads/opportunities
@@ -43,10 +43,10 @@ chatter.
 Both will update if the viewers comes back to your website and visits
 more pages.
 
-.. image:: media/prospect_visits04.png
+.. image:: prospect_visits/prospect_visits04.png
    :align: center
 
-.. image:: media/prospect_visits05.png
+.. image:: prospect_visits/prospect_visits05.png
    :align: center
 
 The feature will not repeat multiple viewings of the same pages in the