Add RFC 9253 compatibility

[niccokunzmann]

Closes issue

Replace ISSUE_NUMBER with the issue number that your pull request fixes. Then GitHub will link and automatically close the related issue.

• Closes RFC 9253 - iCalendar Relationships #658

Description

This intends to solve the issue

Checklist

• add parameter types "# parameter types (luckily there is no name overlap)"
• add to new()
• figure out why the type hints do not work for the properties like Event.conferences and Event.links
• 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 Running Tests.
• I've added or edited documentation, both as docstrings to be rendered in the API documentation and narrative documentation, as necessary.
• I've added myself to docs/credits.rst as a contributor in this pull request or have done so previously.

Still TODO:

• https://icalendar--941.org.readthedocs.build/en/941/reference/api/icalendar.html#icalendar.vUri.GAP timedelta should be clickable.
• timezone link is sometimes not available - https://github.com/collective/icalendar/actions/runs/19035348837/job/54358219688#step:6:299: check validity and then auto-accept it (ignore)

  (reference/api/icalendar.timezone.windows_to_olson: line    8) broken    https://www.unicode.org/cldr/cldr-aux/charts/29/supplemental/zone_tzid.html - ('Connection aborted.', ConnectionResetError(104, 'Connection reset by peer'))
  

Documentation Review guidance

Classes modified:

• https://icalendar--941.org.readthedocs.build/en/941/reference/api/icalendar.html#icalendar.vUri
• https://icalendar--941.org.readthedocs.build/en/941/reference/api/icalendar.cal.html#icalendar.cal.Component.links
• https://icalendar--941.org.readthedocs.build/en/941/reference/api/icalendar.cal.html#icalendar.cal.Component.concepts
• https://icalendar--941.org.readthedocs.build/en/941/reference/api/icalendar.cal.html#icalendar.cal.Component.related_to
• https://icalendar--941.org.readthedocs.build/en/941/reference/api/icalendar.html#icalendar.vText

Additional information

www.rfc-editor.org/rfc/rfc9253.html

[coveralls]

Pull Request Test Coverage Report for Build 19035348858

Details

• 462 of 464 (99.57%) changed or added relevant lines in 23 files are covered.
• No unchanged relevant lines lost coverage.
• Overall coverage increased (+0.07%) to 97.942%

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Changes Missing Coverage	Covered Lines	Changed/Added Lines	%
src/icalendar/attr.py	72	74	97.3%

Totals
Change from base Build 19034014329:	0.07%
Covered Lines:	8551
Relevant Lines:	8722

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💛 - Coveralls

[niccokunzmann]

This is ready for review when you have time. No pressure ;)