Add docs for distros supporting native builds

[danielrainer]

Document on which distros we aim to support building fish using only dependencies packaged for the distro. The intention is providing clarity about which dependency versions can be used safely.

For now, this only contains Rust versions, but it might also be useful for other dependencies, like Python versions. Once we clearly indicate distro version support, it becomes significantly easier to figure out if a dependency version can be changed.

Inspired in part by discussion https://github.com/fish-shell/fish-shell/discussions/11584

TODOs:

• [ ] Replace the stubs by fish's actual support policy
• [ ] Track the Rust versions of all supported distros with references.
• [ ] I'm not that happy with "native builds", since that is often used with a somewhat different meaning. Do we have better ideas?
• [ ] Do we want similar tracking for other dependencies, like Python?
• [ ] Can we get decent table formatting? Using list-table results in GitHub just not rendering the table.
• [ ] Location of these docs: I just created a top-level file to get started. Is there a better location (and file name)?