Overhaul Main Workflow Docs

[ivdiazsa]

This PR aims to make the experience of the Workflow documentation of the runtime repo simpler and more accessible to the general public.

A summary of the changes made in this PR:

• Overhaul the main Workflow README to cover all the basics right off the bat so users and contributors get a solid idea on how to begin working on the repo.
• Condensed all the individual OS instructions for building CoreCLR into a single README, as they contained a lot of duplicate information that resulted in more clicks, more searching, and potentially more confusing.
• Added information to the Building CoreCLR README about how to work in conjunction with other subsets and linking to the corresponding docs.
• Updated a few pieces of information to better match today's status of the repo.
• Rearranged and rephrased a few things in the requirements docs so that the instructions flow more naturally with the workflow one would follow on a new machine.
• Overall rephrased some instructions so that they roll off the tongue more smoothly when reading them and are easier to understand.

Ideas for follow-up work (in this PR and/or other subsequent PR's):

• Merge the CoreCLR and Libraries cross-building docs, as they have lots of things in common.
• Maybe we can make a general building doc and only defer to helper docs the things unique to CoreCLR, Libraries, and Mono.
• Start overhauling other sectors of the documentation such as testing and debugging like this PR does. Make sure the information is up to date, avoid excessive redirections to other docs, rephrase as necessary for a more natural tone, etc.

[lambdageek]

I think it's useful right off the bat to establish that for some workflows cross-compilation is the expected approach.

[mikelle-rogers]

Thanks for putting this together!