Migrate Gerrit guide to source docs.

This is just a straight import of
https://www.chromium.org/developers/gerrit-guide/

(For the Contributing docs, this should be the canonical location, as
opposed to the website.)

I will clean this up a bit afterward.

Bug: 404613530
Change-Id: I40aee5f747dd7ed1c5c5743b5590cf2733f06620
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/6378375
Reviewed-by: Josip Sokcevic <[email protected]>
Reviewed-by: Dirk Pranke <[email protected]>
Commit-Queue: Allen Li <[email protected]>
Cr-Commit-Position: refs/heads/main@{#1437088}
diff --git a/docs/contributing.md b/docs/contributing.md
index ace739f..1b16bf6 100644
--- a/docs/contributing.md
+++ b/docs/contributing.md
@@ -86,7 +86,7 @@
 
 ## Initial git setup
 
-1. Set up [Gerrit access](https://www.chromium.org/developers/gerrit-guide/).
+1. Set up [Gerrit access](gerrit_guide.md).
 2. Tell git about your name, email and some other settings.
    ```
    git config --global user.name "My Name"