Clarify what an identity server is in the docs

This is an issue for half of https://github.com/matrix-org/matrix-doc/issues/546 (the spec also needs to clarify it).

Having skimmed the FAQ, this still seems like a real issue. A header like "What is an identity server?" would be nice.

Start by reading the FAQ and the related matrix-org/matrix-doc#546 issue to understand the documentation gap. Locate the FAQ section where identity servers are introduced and add a clear “What is an identity server?” heading and explanation. Done means the concept is clarified in the website docs without attempting the separate spec work.

Written by the indexing model from the issue text.