Improve docstrings and type hints (Part 2).

[desilinguist]

• chore(req): revert sphinx to <6.0. The sidebar does not work properly with sphinx>=6.0.
• chore: ignore E501 errors in featureset.py.
• chore: fix docstrings and types in skll.data. Also refactored the keyword arguments for many writers to be more explicit rather than using **kwargs everywhere.
• docs: improve layout of api/data.rst. Use individual autoclass items instead of one automodule item to make sure that Reader and Writer base classes are shown before all the other classes.

To review this, please look at the built documentation and check that under the "API documentation" > "data Package`":

• All parameters have a docstring.
• All type hints have clickable types or type hints.
• The default values are all specified as default = blah.
• All other changes look okay.

This is part of #647.

[codecov[bot]]

Codecov Report

Patch coverage: 93.10% and project coverage change: -0.03 :warning:

Comparison is base (7709aa4) 95.21% compared to head (bc08d5d) 95.18%.

Additional details and impacted files

```diff @@ Coverage Diff @@ ## main #738 +/- ## ========================================== - Coverage 95.21% 95.18% -0.03% ========================================== Files 29 29 Lines 3532 3533 +1 ========================================== Hits 3363 3363 - Misses 169 170 +1 ``` | [Impacted Files](https://app.codecov.io/gh/EducationalTestingService/skll/pull/738?src=pr&el=tree&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=EducationalTestingService) | Coverage Δ | | |---|---|---| | [skll/data/dict\_vectorizer.py](https://app.codecov.io/gh/EducationalTestingService/skll/pull/738?src=pr&el=tree&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=EducationalTestingService#diff-c2tsbC9kYXRhL2RpY3RfdmVjdG9yaXplci5weQ==) | `100.00% <ø> (ø)` | | | [skll/data/featureset.py](https://app.codecov.io/gh/EducationalTestingService/skll/pull/738?src=pr&el=tree&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=EducationalTestingService#diff-c2tsbC9kYXRhL2ZlYXR1cmVzZXQucHk=) | `91.57% <ø> (ø)` | | | [skll/data/readers.py](https://app.codecov.io/gh/EducationalTestingService/skll/pull/738?src=pr&el=tree&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=EducationalTestingService#diff-c2tsbC9kYXRhL3JlYWRlcnMucHk=) | `90.42% <ø> (ø)` | | | [skll/data/writers.py](https://app.codecov.io/gh/EducationalTestingService/skll/pull/738?src=pr&el=tree&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=EducationalTestingService#diff-c2tsbC9kYXRhL3dyaXRlcnMucHk=) | `93.20% <93.10%> (-0.46%)` | :arrow_down: |

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