[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/sales/subscriptions/reporting/subscription_reports.rst

@@ -16,7 +16,7 @@ concepts to the proper understanding of the following reports:
   to estimate the coming year's performance. However, this estimation does not take variations and
   growth into account.
 
-.. image:: media/difference-between-MRR-and-ARR.png
+.. image:: subscription_reports/difference-between-MRR-and-ARR.png
   :align: center
   :alt: Difference between MRR and ARR in Odoo Subscriptions
 
@@ -43,7 +43,7 @@ concepts to the proper understanding of the following reports:
        - | Therefore, the 56$ of MRR difference out of the initial 600$ causes a **Revenue Churn**
          | of 9,33%.
 
-       .. image:: media/difference-between-logo-churn-and-revenue-churn.png
+       .. image:: subscription_reports/difference-between-logo-churn-and-revenue-churn.png
           :align: center
           :alt: Difference between logo churn and revenue churn in Odoo Subscriptions
 
@@ -66,7 +66,7 @@ can choose *Quantity*, *Recurring Price*, *Yearly Recurring Revenue* and *Count*
 *Quantity* is added. This way, you can review both of these measures at the same time. You can even
 *Group By Start Date* and, more precisely, by *Week*, to get a clear view of your report.
 
-.. image:: media/subscriptions-analysis-report.png
+.. image:: subscription_reports/subscriptions-analysis-report.png
   :align: center
   :alt: Subscriptions analysis report in Odoo Subscriptions
 
@@ -78,7 +78,7 @@ Go to :menuselection:`Subscriptions --> Reporting --> Retention`. The default me
 *Monthly Recurring Revenue* was chosen, and the *Month* periodicity remains intact.
 By using these criteria, you can see the progression of the retention from its start.
 
-.. image:: media/retention-analysis-report.png
+.. image:: subscription_reports/retention-analysis-report.png
   :align: center
   :alt: Retention analysis report in Odoo Subscriptions
 
@@ -91,7 +91,7 @@ different KPIs: *Monthly Recurring Revenue*, *Net Revenue*, *Non-Recurring Reven
 information on subscriptions, companies, and sales teams. This is useful if you are looking for
 specific information.
 
-.. image:: media/revenue-KPIs-report.png
+.. image:: subscription_reports/revenue-KPIs-report.png
   :align: center
   :alt: Revenue KPIs report in Odoo Subscriptions
 
@@ -100,7 +100,7 @@ data, which is the typical scenario for a new business. But, as your company gro
 these graphs get populated with more and more data. Once again, you can filter these specific KPIs
 on subscriptions, companies, and sales teams.
 
-.. image:: media/detailed-MRR-report.png
+.. image:: subscription_reports/detailed-MRR-report.png
   :align: center
   :alt: Detailed MRR report in Odoo Subscriptions
 
@@ -112,7 +112,7 @@ summary of your *Monthly Recurring Revenue*, *Non-Recurring Revenue*, *Subscript
 and *Non-Recurring Invoices* for each of your salespeople. You can choose the period you want to
 apply and the salesperson you want to analyze.
 
-.. image:: media/salesperson-dashboard-report.png
+.. image:: subscription_reports/salesperson-dashboard-report.png
   :align: center
   :alt: Salesperson dashboard report in Odoo Subscriptions