[Chapel Merge] Add documentation for the unstable keyword

Branch: refs/heads/main
Revision: 931db63
Author: lydia-duncan
Link: Unavailable
Log Message:

Merge pull request #20612 from lydia-duncan/docUnstable

Add documentation for the unstable keyword
[reviewed by @mppf]

Lists the documentation in the Developer section. Recommends using the new
keyword, but also mentions chpl__warnUnstable and compilerWarning as an
alternative if necessary.

Passed make docs locally

Modified Files:
A doc/rst/developer/bestPractices/Unstable.rst

M doc/rst/developer/bestPractices/index.rst

Compare: https://github.com/chapel-lang/chapel/compare/57e73f11a9d3...931db63bf7f5