docs: Add FAQ for keeping changes to tracked files

[jennings]

(I'm fine with having this whole PR changed or even rejected. Since this is a docs change, I figured it was easiest to just make the PR rather than trying to pre-approve it in chat.)

The question "How do I avoid committing changes to files?" comes up a lot in chat, and the solution is not obvious. It will be useful to have a description with an example we can link to.

The wording of the similar question "How can I keep my scratch files in the repository?" was tweaked to emphasize the difference between keeping untracked files in the workspace and keeping changes tracked files out of published history.

Checklist

If applicable:

• I have updated CHANGELOG.md
• I have updated the documentation (README.md, docs/, demos/)
• I have updated the config schema (cli/src/config-schema.json)
• I have added tests to cover my changes

[PhilipMetzger]

While I don't think this will be rejected, as it certainly got popular as a question. There's just some word forgery and simplifications left.

[ilyagr]

Thank you! I think this is useful to add.

I was mildly worried it might be distracting (and if others people feel stronger about this, feel free to chime in), but if the FAQ gets too long, we could always split it into sections with these two questions in its own section.

[PhilipMetzger]

LGTM, I'd wait for another round of feedback from Ilya though.

[ilyagr]

This PR might become out of date once/if #4338 goes in.

[ilyagr]

Actually, on second thought, only the tiny part of this PR that has to do with the "scratch files" question overlaps with #4338, so I may have spoken too soon..

[jennings]

I just noticed that the next question already in the FAQ is "How can I keep local changes around, but not use them for Pull Requests?". That answer recommends doing either:

1. Rebasing the branch on top of the private commit, then rebasing back on top of main before pushing
2. Rebase the private commit on top of the branch, then jj new

This PR gives a third option, creating a merge commit. This is the technique I've seen recommended in Discord lately. I think three options is too many for someone relatively new to Jujutsu, I think we should be more prescriptive until they are comfortable. So I removed the original question entirely. I'm open to changing this decision if people don't like it.

[ilyagr]

This looks pretty good to me. Thank you!

I just noticed that the next question already in the FAQ is "How can I keep local changes around, but not use them for Pull Requests?".

Replacing that question seems to me like it works quite well.