This index file lets us very easily write a generic capture sample.
Previously the dev had to maintain a list of multiple sources files.
Potentially hundreds. By writing the source file list to an index file
we can load this easily in GN as a variable and plug that into the
sources.

Also updates docs.

Bug: angleproject:3611
Change-Id: I69ba961e271d6d13d06ae01c89a0605a6fd725ec
Reviewed-on: https://chromium-review.googlesource.com/c/angle/angle/+/1902189
Commit-Queue: Jamie Madill <jmadill@chromium.org>
Reviewed-by: Cody Northrop <cnorthrop@google.com>