Improve documentation

[tsalo]

Closes none. One slightly, but not meaningfully, breaking change is that the CuBIDS class is no longer accessible through cubids (i.e., you can't do from cubids import CuBIDS). Instead, you need the submodule now (from cubids.cubids import CuBIDS).

Changes proposed:

• Add a glossary that defines some of the core terms.
• Add API page.
• Split out CLI documentation from general usage walkthrough page.
• Remove duplicate README.rst in docs folder.
• Go through documentation and try to split sentences across lines on punctuation.
• Add a section to README on what CuBIDS is not meant to do.
• Add links from API documentation to GitHub code.

[codecov-commenter]

Codecov Report

Attention: 2 lines in your changes are missing coverage. Please review.

Comparison is base (0988440) 62.02% compared to head (597b5e0) 62.13%.

Files	Patch %	Lines
cubids/__about__.py	80.00%	2 Missing :warning:

Additional details and impacted files

```diff @@ Coverage Diff @@ ## main #290 +/- ## ========================================== + Coverage 62.02% 62.13% +0.11% ========================================== Files 8 9 +1 Lines 1572 1582 +10 ========================================== + Hits 975 983 +8 - Misses 597 599 +2 ```

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