Repo governance

Hello @fmeum, @jsharpe, @rrbutani,

Now that the repo has moved over to a more communal organization and is in a good shape, it would be a good time to set up some repo governance and contribution guidelines for the long term. I did set up some basic repository rulesets, but, e.g. have not enforced code reviews. All of you have repo admin permissions, so you can manage these things yourselves. It will be very kind of you if one or more of you can take over as [maintainers](https://github.com/bazelbuild/bazel-central-registry/blob/main/modules/toolchains_llvm/metadata.json#L5) for the registry module.

On a more personal note, I am permanently recusing myself from this project now. It has been 4 years since I actively worked with Bazel rules, and I am not using Bazel in my work anymore. I am also not the best person to know about C++ compiler toolchains, having last written C++ casually in 2017. I hope I am leaving things in a state where it becomes easy for you and future contributors to provide a (mostly) hermetic toolchain for C++ and Bazel users.

Thanks for all the support and keeping this project alive.

Start by reviewing the repository rulesets mentioned in the issue and the existing contribution guidance, then inspect the maintainer information for the registry module. Define the governance and code-review expectations, document the contribution process, and confirm that maintainers have taken ownership of the module.

Written by the indexing model from the issue text.