docs: add DocETL, Kotaemon, spaCy integrations; minor docs improvements

[vagenas]

Checklist:

• [ ] Documentation has been updated, if necessary.
• [ ] Examples have been added, if necessary.
• [ ] Tests have been added, if necessary.

[mergify[bot]]

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🟢 Enforce conventional commit

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Make sure that we follow https://www.conventionalcommits.org/en/v1.0.0/ - [X] `title ~= ^(fix|feat|docs|style|refactor|perf|test|build|ci|chore|revert)(?:\(.+\))?:`