Document 7.x branch

[niccokunzmann]

Closes issue

This documents the branch 7.x.

Checklist

• I've added a change log entry to CHANGES.rst.
• I've added or updated tests if applicable.
• I've run and ensured all tests pass locally by following Run tests.
• I've added or edited documentation, both as docstrings to be rendered in the API documentation and narrative documentation, as necessary.

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📚 Documentation preview 📚: https://icalendar--1207.org.readthedocs.build/

[coveralls]

Pull Request Test Coverage Report for Build 22274488673

Details

• 0 of 0 changed or added relevant lines in 0 files are covered.
• No unchanged relevant lines lost coverage.
• Overall coverage remained the same at 98.081%

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Totals
Change from base Build 22274187028:	0.0%
Covered Lines:	11418
Relevant Lines:	11635

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💛 - Coveralls

[stevepiercy]

No change log needed for this trivial maintenance change. Will merge when green.

However, see also #1213 (comment) for a proposal to reorganize this and similar things.