[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