[Flang] Add GettingInvolved documentation page and sidebar.
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
Reviewed By: richard.barton.arm
Differential Revision: https://reviews.llvm.org/D87270