Expand Matrix URI scheme actions to include action=knock

**Is your feature request related to a problem? Please describe.**

Matrix URI scheme currently only mentions actions join and chat.

* https://spec.matrix.org/latest/appendices/#matrix-uri-scheme

**Describe the solution you'd like**

I would like there to be an action knock too, to communicate to developers that this is a usecase that should be handled as knocking has been part of the specification since 1.1

**Describe alternatives you've considered**

One option would be communicating to clients to knock if join fails.

**Additional context**

Nheko handles the knock action (and in room gave me a permission to open this issue): https://github.com/Nheko-Reborn/nheko/commit/6f8babc80be7883277c3b41dd2ff2baa074f1521

Start with the Matrix URI scheme appendix linked in the issue and review how the existing join and chat actions are described. Update the documented actions to include knock, using the Matrix specification's existing knocking behavior as context, and verify that the appendix clearly communicates this supported use case.

Written by the indexing model from the issue text.