gh-35734: Reference manual: Show modularized sagelib packages separately
<!-- Please provide a concise, informative and self-explanatory title.
-->
<!-- Don't put issue numbers in the title. Put it in the Description
below. -->
<!-- For example, instead of "Fixes #12345", use "Add a new method to
multiply two integers" -->
### :books: Description
<!-- Describe your changes here in detail. -->
Packages such as **sagemath-categories** are currently mixed with
optional SPKGs in the reference guide.
We separate them out as a separate category.
We also move the section on `Feature`s right next to the package list.
#35749 and follow-ups will interlink these sections.
Preview:
- https://deploy-preview-35734--sagemath-
tobias.netlify.app/reference/spkg/index.html#packages-of-the-
modularized-sage-library
- https://deploy-preview-35734--sagemath-
tobias.netlify.app/reference/spkg/index.html#runtime-detectable-
features-and-conditional-doctests
<!-- Why is this change required? What problem does it solve? -->
<!-- If this PR resolves an open issue, please link to it here. For
example "Fixes #12345". -->
- Part of #29705
<!-- If your change requires a documentation PR, please link it
appropriately. -->
### :memo: Checklist
<!-- Put an `x` in all the boxes that apply. It should be `[x]` not `[x
]`. -->
- [x] The title is concise, informative, and self-explanatory.
- [x] The description explains in detail what this PR is about.
- [ ] I have linked a relevant issue or discussion.
- [ ] I have created tests covering the changes.
- [ ] I have updated the documentation accordingly.
### :hourglass: Dependencies
<!-- List all open PRs that this PR logically depends on
- #12345: short description why this is a dependency
- #34567: ...
-->
<!-- If you're unsure about any of these, don't hesitate to ask. We're
here to help! -->
URL: https://github.com/sagemath/sage/pull/35734
Reported by: Matthias Köppe
Reviewer(s): Kwankyu Lee, Matthias Köppe
