docs: Add pointer nullability annotation guidelines

Adds recommendations for explicitly annotating nullable pointers with
absl_nullable and absl_nonnull to prevent accidental use of invalid
null values.

No-try: true
Bug: none
Change-Id: Ic27cbfe4522c16fbd0a4206e93e0e7cf15bf0c0e
Reviewed-on: https://webrtc-review.googlesource.com/c/src/+/448500
Reviewed-by: Danil Chapovalov <danilchap@webrtc.org>
Commit-Queue: Tomas Gunnarsson <tommi@webrtc.org>
Cr-Commit-Position: refs/heads/main@{#46882}