Update stale/broken links.  Use consistent link style:
 - links between our files should relative
 - omit filename for links within same file
 - don't specify heading id
Don't use same heading twice in one file because the markdown compilers
on googlesource.com and github have different ways of disambiguating them,
so this is the easiest way to have links work on both sites.

BUG=angleproject:1569

Change-Id: Iefd5ab8014d582a017f64e383f301ea0b8e60433
Reviewed-on: https://chromium-review.googlesource.com/789445Reviewed-by: Yuly Novikov <ynovikov@chromium.org>
Reviewed-by: Geoff Lang <geofflang@chromium.org>
Commit-Queue: Frank Henigman <fjhenigman@chromium.org>