Merge extension support docs. We previously had an existing document in doc/ExtensionSupport.md. This merges the new SupportedExtensions.md into the old location and includes a bit more info on EGL extension support. Bug: angleproject:6379 Change-Id: Ibb60a9e9db9200abe609383531131903ca698e60 Reviewed-on: https://chromium-review.googlesource.com/c/angle/angle/+/3197770 Reviewed-by: Jonah Ryan-Davis <jonahr@google.com> Commit-Queue: Jamie Madill <jmadill@chromium.org>