unify wording for times and dates

- Types: Duration, Date, DateTime
- always use createdAt, updatedAt (not On etc.)

[ci skip]
mark start and due date writable

[ci skip]
add missing ""

[ci skip]
Merge pull request #2234 from opf/feature/specify_patchable_dates

Specify patchable dates
clean tables

- removed examples
- ISO 8601 information moved to "basic objects" section, also linking to Wikipedia

[ci skip]
some more minor cleanup

[ci skip]
Test that collections adhere to the specifiction
Implement collections specification
unify wording to x > 0

[ci skip]
Remove obsolete available statuses endpoint

With the availibility of those statuses in the WP form this became obsolete.
formatting
Merge pull request #2237 from opf/feature/clean_spec_tables

Cleanup API specification tables
Merge pull request #2231 from opf/fix/17403_implement_collection_specification

17403 implement collection specification