Updated process model documentation

[Charlie Reis]

Hi folks!

  Just wanted to send a broader note that we've added some Markdown documentation for Chromium's process model, which also covers some of the high level concepts from Site Isolation.  You can find it at:

https://chromium.googlesource.com/chromium/src/+/main/docs/process_model_and_site_isolation.md

The document covers the general design of how Chromium chooses which renderer processes to use for which pages, as well as commonly asked questions about process reuse, process locks, and special cases.  It replaces some of the out-of-date descriptions that used to be at https://www.chromium.org/developers/design-documents/process-models/.  Thanks to everyone who provided feedback, and hope it proves useful!

Charlie Reis (and the Chrome Security Architecture team)