MNT: add documentation issue template

[ianhi]

Add a documentation issue template to make less friction for someone to report a docs issue/improvement.

This is same content I added to icechunk (and other repos farther back in time): https://github.com/earth-mover/icechunk/blob/main/.github/ISSUE_TEMPLATE/documentation.md?plain=1

but yamlified for consistency

[max-sixty]

I'm generally not so enthusiastic on these sorts of issues fwiw

if it's an atomic project such as "build a demo" or even #11031, then it's reasonable, it's actionable

but if it's "the docs on indexes vs coords is a bit confusing", then I'm not sure the issue does much — it's rare that someone who is browsing issues decides to contribute docs — so we end up with lots of issues that are closer to opinions than MCVEs

possibly with LLMs this becomes much easier though? (and am ofc fine with merging...)