style(docs): make permanent headerlink reappear in nice look #8642

[poikilotherm]

What this PR does / why we need it:
Being able to copy permalinks from the guides without scrolling to top first makes using the guides much easier.

Which issue(s) this PR closes:

Closes #8642

Special notes for your reviewer:
THIS PR IS BASED ON #8650 / #8651, so these should be merged first. Thx!

If you don't like the color, the symbol or the size, please propose changes. Please note that the anchors only appear during "mouse over" of a heading and are hidden otherwise (see below).

Suggestions on how to test this:
Take a look at the screenshot or take a look at the RTD based preview (click on the "Details" link within the checks status box)

Does this PR introduce a user interface change? If mockups are available, please link/include them here:

Is there a release notes update needed for this change?:
Nope.

Additional documentation:
None.

[pdurbin]

or take a look at the RTD based preview (click on the "Details" link within the checks status box)

Or here's a direct link to the "mail domain" example in the screenshot: https://dataverse-guide--8647.org.readthedocs.build/en/8647/admin/mail-groups.html

[poikilotherm]

@pdurbin who needs to review this? Shall this really wait until next sprint? (Dunno why it wasn't added to review this week 🤷 )

[scolapasta]

@poikilotherm this did not make into this sprint because we are trying a new process where new external PRs don't immediately intterrupt the currrent sprint cadence. For now, we will take x PRs and put them into review at the beginning of a sprint. For this sprint, x was 10 and I tried to choose 1-2 for different contributors. In this case, this PR said it was dependent on the other and since I didn't know when it would get merged, I opted to leave it out.

That said, mid sprint, if we pur moving along and have merged (or removed, do to non trivial rework being needed) the majority of the external PRs, there may be room to move some additional ones. This one is. a candidate for that. If it doesn't I will make sure it is definitely in the next sprint.

[pdurbin]

I ran the code and I'm approving this but I'd like to note that I got this error when running make html:

$ make html
sphinx-build -b html -d build/doctrees  -W source build/html
Running Sphinx v3.5.4
loading translations [en]... done

Extension error:
Could not import extension sphinxcontrib.icon (exception: No module named 'sphinxcontrib.icon')
make: *** [html] Error 2

To me, this means we should all run pip install -r requirements.txt to pick up the new "sphinx-icon" dependency.

Our docs on how to get Sphinx installed were woefully out of date so I updated them to indicate that requirements.txt should be used to install dependencies. I also did some various cleanup while I was in there.

I must say that the new anchors are very nice! I plan to use them basically all the time. Thanks, @poikilotherm!

[kcondon]

Tested, this works but waiting on continuous integration.

[pdurbin]

To me, this means we should all run pip install -r requirements.txt to pick up the new "sphinx-icon" dependency.

Now that we've merged this PR, I just emailed the dev list about this: https://groups.google.com/g/dataverse-dev/c/fZpTQYQKR0g/m/DQTARER7AwAJ