Add documentation that lists custom well-known properties

Currently Element supports two custom properties in the client well-known file, which are documented in two different files:

* https://github.com/vector-im/element-web/blob/develop/docs/jitsi.md
* https://github.com/vector-im/element-web/blob/develop/docs/e2ee.md

It would be useful to know from a glance whether a specific feature depends from a well-known property. i.e., it would be nice to have a document that lists existing ones, and this document could link to each property's individual doc file for more documentation.

Start by reviewing docs/jitsi.md and docs/e2ee.md in the linked Element Web repository to identify the documented custom well-known properties. Add a document that lists the existing properties and links to each property's detailed documentation. Done means the properties can be found from one overview document.

Written by the indexing model from the issue text.