GitLab

Replace most of sheriff.md with a link to internal docs.

I've left the section about sheriffing philosophy, since that's also useful for non-sheriffs.

Bug: 1095911
Change-Id: I9e97cc1951e5cc96fe9aa8ccc42e14c875f0f0b1
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2274481
Commit-Queue: Joey Scarr <jsca@chromium.org>
Reviewed-by: Elly Fong-Jones <ellyjones@chromium.org>
Reviewed-by: Dirk Pranke <dpranke@google.com>
Cr-Commit-Position: refs/heads/master@{#784319}