CONTRIBUTING.md is unclear about standard library

[araujoms]

I just submitted a PR to change something in LinearAlgebra (#53186). To do that I followed the instructions here about how to contribute to the standard library, which didn't work. @jishnub then kindly remarked that I should instead follow the instructions for contributing to the core functionality or base libraries, which did work.

Either the instructions are incorrect or LinearAlgebra is not part of the standard library, despite being in the stdlib folder. Either way I think the documentation should be updated.

[ViralBShah]

Based on your experience, do you have a set of instructions that you can provide in a PR to CONTRIBUTING.md?

[vtjnash]

Sounds like we should try to clarify here the list of stdlibs that can be upgraded that way (e.g. make print-STDLIBS_WITHIN_SYSIMG), and those that cannot? as well as those that are in a separate repo (and can use USE_DEPS=<names> in Make.user) vs those that are edited directly here?

[vtjnash]

Hid a side-conversation about LinearAlgebra having a separate repo, since the resolution was that we deleted it now, and it detracts from the main conversation here about docs