chore(docs): move deprecated platforms information around

chore(docs): move deprecated platforms information around

We used to have a dedicated page for deprecated platforms information.
This document contained 2 pieces of information:

 a) the process for deprecating a platform port;
 b) the list of deprecated platforms to this day.

I think it makes more sense to move b) to the platforms ports landing
page, such that it is more visible.

This also has the nice effect to move the 'Deprecated platforms' title
as the last entry of the 'Platform ports' table of contents, like so:

 - Platform ports
   - 1. Allwinner ARMv8 SoCs
   - 2. Arm Development Platforms
     ...
   - 39. Broadcom Stingray
   - Deprecated platforms

instead of it being lost in the middle of supported platform ports.

Regarding a), this gets moved under the "Processes & Policies" section.
More specifically, it gets clubbed with the existing platform
compatibility policy. The combined document gets renamed into a
"Platforms Ports Policy" document.

Change-Id: I6e9ce2abc68b8a8ac88e7bd5f21749c14c9a2af6
Signed-off-by: Sandrine Bailleux <sandrine.bailleux@arm.com>