Better document commit bit access and what it entails

With the Nix team expanding (@lovesegfault, @Radvendii, @lisanna-dettwyler), it seems like it's about time to better document the process and have it be more publically visible. One existing frustrations is that the current process isn't obvious to people outside the team and it's unclear how a regular contributor can request to be added to the team. The existing process is consensus driven based on prior interactions with the contributors and their technical contributions. I believe we'd benefit from adopting a process similar to nixpkgs/LLVM with the commit bit being requested on the issue tracker with a more transparent vouching/endorsement from the existing team members with a public trail. Another thing that's been on my mind is that there might be a place for a 2-tiered system, since commit bit doesn't necessarily have to entail the full responsibilities of a nix team member (having access and triaging/fixing security issues e.t.c.). There are some components which could benefit from this (also -c bindings, which are part of the repo but whose owners and regular contributors don't necessarily have to be all part of the Nix team). There are other widely used projects closely tied to the C++ API (think nix-eval-jobs https://github.com/NixOS/org/issues/227#issuecomment-3759682560), which would be easier to maintain if they were part of the source tree - the intersection of people contributing to Nix and being proficient in the Nix C++ API is mostly a circle. There's some existing process we could look into for some examples: - https://llvm.org/docs/DeveloperPolicy.html#obtaining-commit-access - nixpkgs https://github.com/NixOS/nixpkgs-committers