Check Correct Style and Spelling in Docstrings

The Vale spellchecker helps us already with writing documentation that is not misspelled and has a consistent style.

It is possible to configure Vale to also check the Python docstrings. They are part of the documentation too. Actually a huge part.

The following pull request showed considerable promise to resolve this issue, but was abandoned by the author. You could reopen the pull request and continue work on it.

https://github.com/collective/icalendar/pull/1111

To close this issue:

- [ ] configure Vale to run on the Python documentation
- [ ] It is not needed to run on test code or comments
- [ ] create a pull request and resolve the spelling mistakes
- [ ] possibly extend the accept list and reject lists of words

See also:
- https://vale.sh/docs/formats/code
- https://github.com/collective/icalendar/pull/979#discussion_r2553701328_

Start by reviewing the abandoned pull request #1111, the existing Vale configuration, and Vale's documentation for checking code formats. Inspect the Python docstrings rather than test code or comments, then use the issue checklist to verify that spelling and style findings are addressed and any necessary accept or reject lists are updated.

Written by the indexing model from the issue text.