Add Monorail -> Google Issue Tracker map.

This CL also creates a docs/monorail-bog-tracker-migration where
we can add more information (if needed) about the migration.

No-Try: True
Bug: None
Change-Id: Ieec36a793daf8e30c6181f7cd595fce922948838
Reviewed-on: https://webrtc-review.googlesource.com/c/src/+/349323
Reviewed-by: Harald Alvestrand <[email protected]>
Commit-Queue: Mirko Bonadei <[email protected]>
Cr-Commit-Position: refs/heads/main@{#42195}

Author: MirkoBonadei