Document new grammar workflow

[malaverdiere]

Now that we merged repositories responsible for grammars in Semgrep, which changed the tooling and steps, contributors need updated documentation.

Thanks for improving Semgrep Docs

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[kopecs]

This looks mostly good but I would drop the numbers from the headings for consistency and double check the formatting, some of it seemed rendered poorly on github.

[abhijna]

Hi @malaverdiere! These docs haven't been touched in a while, so while I was here, I made a few updates to better align them with the Semgrep technical writing style. I also incorporated some of @kopecs's suggestions.

I'll approve this now and can merge it if everything looks good to you.