Add ContentLine import into docs

[kvnshu]

Maybe include from ics.grammar.parse import ContentLine in the Advanced Usage section? Had to go to ics.alarm.studio to find the import

[N-Coder]

Yep, docs will still need quite some work for the new version, thanks for bringing this up!
Could you also post your code for creating the repeating event in the other issue?
This might also be a nice example to have in the docs until we get first-class support.