Documentation: RAS: Add index and address translation section

There are a lot of RAS topic to document, and there are a lot of details
for each topic.

Prep for this by adding an index for the RAS directory. This will
provide a top-level document and table of contents. It also provides the
option to build the RAS directory individually using "make SPHINXDIRS=".

Start a section on address translation. This will be expanded with
details for future translation methods and how they're used in the
kernel.

Move the error decoding topic to its own section. Links to other error
decoding kernel docs will be added.

Signed-off-by: Yazen Ghannam <yazen.ghannam@amd.com>
Signed-off-by: Borislav Petkov (AMD) <bp@alien8.de>
Link: https://lore.kernel.org/r/20240123041401.79812-4-yazen.ghannam@amd.com