limit table of contents in guides to depth of 2

[pdurbin]

What this PR does / why we need it:

As discussed in a couple meetings with the Documentation Working Group, the amount of information you see when clicking on any given guide is overwhelming. Here's an example from the User Guide:

This pull requests limits the depth of the table of contents to two levels. Here's the User Guide as an example:

Which issue(s) this PR closes:

None

Special notes for your reviewer:

The majority of our guides are the old school .rst variety and look the same. Two newer guides are use .md and look a bit different:

• The QA Guide shows "Contents:" in regular font instead of bold. I tried a bit to fix this but it was non-obvious so I gave up. Perhaps someday we'll move all guides from .rst to .md and they will all be consistent.
• The Contributor Guide has a box around the contents. This is because the most of the content is on a single page. This was done be design when then guide was created (see Add a Contributor Guide #10532).

Suggestions on how to test this:

Make sure the depth is 2.

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

No but there are screenshots above.

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

Yes, included.

Additional documentation:

You can preview the change at https://dataverse-guide--11166.org.readthedocs.build/en/11166/