Minor refactor of adding_to_third_party

* Reduced the indentation of headings so that the TOC is more useful.
* Changed 'README.chromium' to a H1 heading to make navigating from the TOC easier when looking for information on a specific field.
* Reformatted [Update Mechanism](https://chromium.googlesource.com/chromium/src/+/800f641fc14175cdf1aa717679906f1ae561d35b/docs/adding_to_third_party.md?pli=1#update-mechanism) section to be more readable.

Live Preview:
https://chromium.googlesource.com/chromium/src/+/800f641fc14175cdf1aa717679906f1ae561d35b/docs/adding_to_third_party.md?pli=1#Autoroll

Change-Id: I1339f5dbcc46cba56e18d1269648ac69b8a7c69d
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/6631790
Reviewed-by: Jiewei Qian <qjw@chromium.org>
Commit-Queue: Jordan Brown <rop@google.com>
Reviewed-by: Rachael Newitt <renewitt@google.com>
Cr-Commit-Position: refs/heads/main@{#1472891}