Document supported min/max gcc/clang versions in Building Grain from Source

[terrablue]

Related to #2315.

For those building themselves, particularly on rolling distributions such as Arch Linux, it might be beneficial to list the supported range of GCC/Clang versions at https://grain-lang.org/docs/getting_grain#Building-Grain-from-Source

That way, using tools like mise, nix or asdf, the user might be able to install another version of the compiler to meet the prerequisites.

[spotandjake]

I think some questions we would need to address are:

• When would these tests run (probably not every commit to main)
• What version range would we want to consider testing/guaranteeing?

Currently, we don't really have a hard rule on what versions we do or don't support. However we test on macos-latest, ubuntu-latest and windows-latest in ci and most of our team does active development on macos so we try and support the bundled gcc/clang distributions bundled with these operating systems actively.