DOC restructure contributing environment guide

[MarcoGorelli]

• closes BUILD: No module named 'pandas._libs.interval' after building container from .devcontainer.json #50134 (Replace xxxx with the GitHub issue number)
• Tests added and passed if fixing a bug or adding a new feature
• All code checks passed.
• Added type annotations to new arguments/methods/functions.
• Added an entry in the latest doc/source/whatsnew/vX.X.X.rst file if fixing a bug or adding a new feature.

Hey @noatamir , thoughts on this? I think it's clearer to split it up into 3 steps:

I've also removed some parts which are arguably unnecessary

[MarcoGorelli]

Just noticed #50144 - let's try to get that one in first

[mroeschke]

Suggested change

	if you modified them or if you did a fetch and merge from ``upstream/main``.
	if you modified any file in `pandas/_libs` or if you did a fetch and merge from ``upstream/main``.

[MarcoGorelli]

there's also some in pandas/io though

[mroeschke]

Oh I didn't know there's some sas cython code there (I think that should be moved to pandas/_libs at some point).

Mainly wanted to clarify here what files constitute C extensions (pyx files)

[MarcoGorelli]

OK I've gone with your suggestion (it does say "for example", so it's fine if it's not exhaustive, especially if the sas files should be moved to pandas/_libs too)

[mroeschke]

LGTM

[MarcoGorelli]

thanks! @natmokval seeing as you went through this not too long ago, reckon it makes it clearer for new contributors?

[MarcoGorelli]

@meeseeksdev please backport to 1.5.x

[lumberbot-app]

Owee, I'm MrMeeseeks, Look at me.

There seem to be a conflict, please backport manually. Here are approximate instructions:

1. Checkout backport branch and update it.

git checkout 1.5.x
git pull

2. Cherry pick the first parent branch of the this PR on top of the older branch:

git cherry-pick -x -m1 70eef55e438823697515e834ee4ac350bdb0aaa7

3. You will likely have some merge/cherry-pick conflict here, fix them and commit:

git commit -am 'Backport PR #50145: DOC restructure contributing environment guide'

4. Push to a named branch:

git push YOURFORK 1.5.x:auto-backport-of-pr-50145-on-1.5.x

5. Create a PR against branch 1.5.x, I would have named this PR:

"Backport PR #50145 on branch 1.5.x (DOC restructure contributing environment guide)"

And apply the correct labels and milestones.

Congratulations — you did some good work! Hopefully your backport PR will be tested by the continuous integration and merged soon!

Remember to remove the Still Needs Manual Backport label once the PR gets merged.

If these instructions are inaccurate, feel free to suggest an improvement.

[MarcoGorelli]

@meeseeksdev please backport to 1.5.x