Improve docstrings and create linkable type hints (Part 1)

[desilinguist]

• Convert all custom types in skll.types to TypeAlias objects.
• Add typing_extensions to requirements.txt since TypeAlias in only in Python 3.10+.
• Clean up doc/conf.py.
• Add skll.types to the API documentation section by creating a new file api/types.rst.
• Add intersphinx mappings for python, numpy, scikit-learn, and pandas.
• Fix docstrings in skll.config module to match scikit-learn style, add type hints to the docstrings, and link type hints to SKLL-internal documentation or to the appropriate external documentation.
• Update doc/requirements.txt to use newer versions of sphinx and sphinx_rtd_theme.

Here's the corresponding documentation built from this branch: https://skll.readthedocs.io/en/647-improve-docstrings-typehints-1/

This is the first PR under #647.

[codecov[bot]]

Codecov Report

Patch coverage: 100.00% and no project coverage change.

Comparison is base (0a4381a) 95.21% compared to head (9ef3f53) 95.21%.

Additional details and impacted files

```diff @@ Coverage Diff @@ ## main #737 +/- ## ======================================= Coverage 95.21% 95.21% ======================================= Files 29 29 Lines 3531 3532 +1 ======================================= + Hits 3362 3363 +1 Misses 169 169 ``` | [Impacted Files](https://app.codecov.io/gh/EducationalTestingService/skll/pull/737?src=pr&el=tree&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=EducationalTestingService) | Coverage Δ | | |---|---|---| | [skll/config/\_\_init\_\_.py](https://app.codecov.io/gh/EducationalTestingService/skll/pull/737?src=pr&el=tree&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=EducationalTestingService#diff-c2tsbC9jb25maWcvX19pbml0X18ucHk=) | `94.75% <ø> (ø)` | | | [skll/config/utils.py](https://app.codecov.io/gh/EducationalTestingService/skll/pull/737?src=pr&el=tree&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=EducationalTestingService#diff-c2tsbC9jb25maWcvdXRpbHMucHk=) | `96.15% <ø> (ø)` | | | [skll/types.py](https://app.codecov.io/gh/EducationalTestingService/skll/pull/737?src=pr&el=tree&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=EducationalTestingService#diff-c2tsbC90eXBlcy5weQ==) | `92.59% <100.00%> (+0.28%)` | :arrow_up: |

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