[libc++][docs] Update contributing docs to reflect the move to GitHub (#69386)

to libc++. If this is your first time contributing, please also read `this document

<https://www.llvm.org/docs/Contributing.html>`__ on general rules for contributing to LLVM.

For libc++, please make sure you follow `these instructions <https://www.llvm.org/docs/Phabricator.html#requesting-a-review-via-the-command-line>`_

for submitting a code review from the command-line using ``arc``, since we have some

automation (e.g. CI) that depends on the review being submitted that way.

If you plan on contributing to libc++, it can be useful to join the ``#libcxx`` channel

on `LLVM's Discord server <https://discord.gg/jzUbyP26tQ>`__.

Looking for pre-existing reviews

================================

Looking for pre-existing pull requests

======================================

Before you start working on any feature, please take a look at the open reviews

to avoid duplicating someone else's work. You can do that by going to the website

where code reviews are held, `Differential <https://reviews.llvm.org/differential>`__,

and clicking on ``Libc++ Open Reviews`` in the sidebar to the left. If you see

that your feature is already being worked on, please consider chiming in instead

of duplicating work!

Before you start working on any feature, please take a look at the open libc++ pull

requests to avoid duplicating someone else's work. You can do that on GitHub by

filtering pull requests `tagged with libc++ <https://github.com/llvm/llvm-project/pulls?q=is%3Apr+is%3Aopen+label%3Alibc%2B%2B>`__.

If you see that your feature is already being worked on, please consider chiming in

and helping review the code instead of duplicating work!

RFCs for significant user-affecting changes

===========================================

Unlike most parts of the LLVM project, libc++ uses a pre-commit CI [#]_. This

CI is hosted on `Buildkite <https://buildkite.com/llvm-project/libcxx-ci>`__ and

the build results are visible in the review on Phabricator. Please make sure

the build results are visible in the review on GitHub. Please make sure

the CI is green before committing a patch.

The CI tests libc++ for all :ref:`supported platforms <SupportedPlatforms>`.

The build is started for every diff uploaded to Phabricator. A complete CI run

takes approximately one hour. To reduce the load:

The build is started for every commit added to a Pull Request. A complete CI

run takes approximately one hour. To reduce the load:

* The build is cancelled when a new diff for the same revision is uploaded.

* The build is cancelled when a new commit is pushed to a PR that is already running CI.

* The build is done in several stages and cancelled when a stage fails.

Typically, the libc++ jobs use a Ubuntu Docker image. This image contains