Fix current version breaking side menu (remove bootstrap, custom css, custom js, custom template)

[braingram]

This PR:

• removes the old version (2019) of bootstrap
• removes the custom css
• removes the empty custom javascript file
• removes the custom template (required to include the above removed items)
• updates a few of the rendered docutils nodes to lists, headers, etc instead of plain text to allow the above changes to have a minimal impact on the rendered docs

The above changes are mostly motivated by an issue where the old version of bootstrap causes the side/navigation menu to refuse to scroll. Making the above changes fixes this issue.

One side-effect of the above changes is the loss of the details button which collapses and expands some portions of the schema description. This seems like a reasonable loss for the benefit of not having to maintain all of the above items (which are currently causing the above described issue).

As the CI here doesn't provide previews for rendered docs I opened a test PR with asdf-standard that uses the source branch for this PR:
asdf-format/asdf-standard#418
the rendered docs for the test PR are here:
https://asdf-standard--418.org.readthedocs.build/en/418/

Here are a few useful comparisons:

• core/asdf:
  • current: https://asdf-standard.readthedocs.io/en/latest/generated/stsci.edu/asdf/core/asdf-1.1.0.html
  • with this pr: https://asdf-standard--418.org.readthedocs.build/en/418/generated/stsci.edu/asdf/core/asdf-1.1.0.html
• core/complex:
  • current: https://asdf-standard.readthedocs.io/en/latest/generated/stsci.edu/asdf/core/complex-1.0.0.html
  • with this pr: https://asdf-standard--418.org.readthedocs.build/en/418/generated/stsci.edu/asdf/core/complex-1.0.0.html

For core/asdf the current docs contain the following:

with details "expanded":

with this PR the docs contain:

Test PRs are also open for stdatamodels and rad:
spacetelescope/stdatamodels#246
spacetelescope/rad#354
The formatting for these is slightly different and are subject to the numerous bugs in the stsci sphinx theme.

[nden]

I think this is fine. I agree overall the benefits outweigh the loss of the details button.

[braingram]

Thanks for the review!