[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/developer/howtos/rdtraining/12_sprinkles.rst

@@ -26,7 +26,7 @@ Inline Views
     **Goal**: at the end of this section, a specific list of properties should be added to the property
     type view:
 
-    .. image:: 12_sprinkles/media/inline_view.png
+    .. image:: 12_sprinkles/inline_view.png
       :align: center
       :alt: Inline list view
 
@@ -101,7 +101,7 @@ Widgets
     **Goal**: at the end of this section, the state of the property should be displayed using a
     specific widget:
 
-    .. image:: 12_sprinkles/media/widget.png
+    .. image:: 12_sprinkles/widget.png
       :align: center
       :alt: Statusbar widget
 
@@ -240,7 +240,7 @@ Form
     - Conditional display of buttons and fields
     - Tag colors
 
-    .. image:: 12_sprinkles/media/form.gif
+    .. image:: 12_sprinkles/form.gif
       :align: center
       :alt: Form view with sprinkles
 
@@ -322,11 +322,11 @@ List
     Additionally, offers and tags will be editable directly in the list, and the availability date will be
     hidden by default.
 
-    .. image:: 12_sprinkles/media/decoration.png
+    .. image:: 12_sprinkles/decoration.png
       :align: center
       :alt: List view with decorations and optional field
 
-    .. image:: 12_sprinkles/media/editable_list.gif
+    .. image:: 12_sprinkles/editable_list.gif
       :align: center
       :alt: Editable list
 
@@ -393,7 +393,7 @@ Search
     and searching on the living area returns results where the area is larger than the given
     number.
 
-    .. image:: 12_sprinkles/media/search.gif
+    .. image:: 12_sprinkles/search.gif
       :align: center
       :alt: Default filters and domains
 
@@ -441,7 +441,7 @@ Stat Buttons
     **Goal**: at the end of this section, there will be a stat button on the property type form view
     which shows the list of all offers related to properties of the given type when it is clicked on.
 
-    .. image:: 12_sprinkles/media/stat_button.gif
+    .. image:: 12_sprinkles/stat_button.gif
       :align: center
       :alt: Stat button