gh-35749: Add style guide / reference for `# optional - sage....` doctest tags, extend `sage -t` and `sage -fixdoctests` for modularization tasks
    
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### 📚 Description

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- Fixing the handling of file-level `# optional` tags.
- Some files were not being doctested; fixing the recently introduced
errors in doctests.
- Implementing block-scoped tags (originally done in PR #35779, merged
here)
- Expanding the documentation on the `# optional` / `# needs` tags used
for modularization purposes, with examples.
- Adding features `sage.modular`, `sage.numerical.mip`,
`sage.rings.complex_double`, `sage.sat`
- The tools `sage -t` and `sage --fixdoctests` receive some new options
for modularization tasks (see added documentation):
   ```
   $ make pypi-wheels
   $ make SAGE_CHECK=yes sagemath-modules
   $ ./sage --fixdoctests --distribution sagemath-modules \
src/sage/combinat/root_system/root_lattice_realization_algebras.py
   ```
   (example uses #35095)
- Suppressing `# optional`/`# needs` of present features in the help
system
.

<!-- Why is this change required? What problem does it solve? -->
Motivated by the sage-devel thread https://groups.google.com/g/sage-
devel/c/utA0N1it0Eo (2023-06)

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example "Fixes #12345". -->
- Resolves #35790
- Resolves #35750
- Part of #29705
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appropriately. -->

### 📝 Checklist

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- [x] I have linked a relevant issue or discussion.
- [ ] I have created tests covering the changes.
- [x] I have updated the documentation accordingly.

### ⌛ Dependencies

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- Depends on #35820 (merged here)
- Vote at https://groups.google.com/g/sage-devel/c/8KZNRBs6F6U (result:
https://groups.google.com/g/sage-devel/c/MtS2u3VbJEo)
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URL: #35749
Reported by: Matthias Köppe
Reviewer(s): Kwankyu Lee, Matthias Köppe