[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#2064

content/applications/finance/accounting/receivables/customer_payments/online_payment.rst

@@ -7,7 +7,7 @@ To make it more convenient for your customers to pay the invoices you issue, you
 allows your customers to see their invoices online and pay directly with their favorite payment
 method, making the payment process much easier.
 
-.. image:: media/online-payment-acquirers.png
+.. image:: online_payment/online-payment-acquirers.png
    :align: center
    :alt: Payment acquirer choice after having clicked on "Pay Now"
 
@@ -30,13 +30,13 @@ After issuing the invoice, click on *Send & Print* and send the invoice by email
 They will receive an email with a link that redirects them to the invoice on their **Customer
 Portal**.
 
-.. image:: media/online-payment-view-invoice.png
+.. image:: online_payment/online-payment-view-invoice.png
    :align: center
    :alt: Email with a link to view the invoice online on the Customer Portal.
 
 They can choose which Payment Acquirer to use by clicking on *Pay Now*.
 
-.. image:: media/online-payment-pay-now.png
+.. image:: online_payment/online-payment-pay-now.png
    :align: center
    :alt: "Pay now" button on an invoice in the Customer Portal.