docs: add warning when rendering preview docs

[SnoopJ]

Description

I had not realized that the unstable documentation was being regularly built against master, and I thought it would be helpful if these preview docs included a warning about their preview-ness as well as a reference to the specific commit they were built against.

The warning added by this changeset is included on all pages using Sphinx's rst_prolog variable, but only rendered when the Sopel version is a pre-release (i.e. .devN) version.

Below is an example of how preview documentation renders (the links points to the relevant commit on GitHub and https://sopel.chat/docs/, respectively)

And here's what the docs look like for a release version:

Checklist

• [x] I have read CONTRIBUTING.md
• [x] I can and do license this contribution under the EFLv2
• [x] No issues are reported by make qa (runs make lint and make test)
• [x] I have tested the functionality of the things this change touches

[SnoopJ]

using subprocess in Python is something I'm very averse to for many reasons.

Is that a generalized aesthetic concern, or are you thinking of specific failure modes here?

An alternative that occurred to me while putting this PR together would be to add a Makefile target for the preview documentation that retrieves the relevant commit information and passes it to Sphinx. I don't know what the specific mechanism would be, but I'm sure we could pass it in.

Edit: per discussion on IRC, I will rewrite to shove as much git wrangling into the Makefile as possible, without error handling if git is not present or if calling git fails.