Adds a new GettingInvolved page to documentation which provides details about
mailing list, chats and calls.
Adds a sidebar page which provides common links on
all documentation pages.
The links include:
- Getting Started
- Getting Involved
- Github Repository
- Bug Reports
- Code Review
Depends on https://reviews.llvm.org/D87242
The heading depths need to be corrected for this to display well in the new index file toctrees. Needs to be # for the top-level section and ## for the subsections.