Add documentation for modifiers

[cbarrete]

cc @cjhopman since you wrote the original RFC :)
Let me know if something is inaccurate.

Related to #846

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[scottcao]

Thanks for doing this. This is a good starting point. I will take this and polish it a bit.

Note the original RFC I wrote has been moved from markdown file to google doc because it's much faster to iterate. However, a pdf copy of it still lives in the repo: https://github.com/facebook/buck2/blob/main/docs/rfcs/cfg-modifiers/modifiers.pdf. The google doc is available for view as well but you do need to request permission to see it.

[scottcao]

The google doc I was referring to lives in https://docs.google.com/document/d/1yfb2mwnsphGiz9FhftkIGwMorK8is1qq3f_gYOEmkMA/.

[cbarrete]

Thanks for doing this. This is a good starting point. I will take this and polish it a bit.

Thanks! I'm happy to get the ball rolling on a bunch of documentation subjects, e.g.:

• Documentation on toolchains is lacking #666
• Documentation on labels is lacking #667

I have a decent intuition for those, but nothing that would necessarily be completely technically correct. Given a few pointers I wouldn't mind opening a PR, even if it needs some tweaking from someone at Meta.

Regarding the RFC, I actually linked it in the "More information" section of the main page. It was quite useful for me to get started with modifiers, but:

• It's not currently exposed on https://buck2.build/docs
• It's not clear to casual readers how up to/out of date the rendered PDF is

[scottcao]

Yeah the fact the pdf is not available on the website is an issue. I will make a note to look into it. The pdf is reasonably up to date but I will publish a new version of it shortly.

I have a decent intuition for those, but nothing that would necessarily be completely technically correct. Given a few pointers I wouldn't mind opening a PR, even if it needs some tweaking from someone at Meta.

I think this would be great. Happy to assist here with pointers or tweaking the PR.