Factor out Icons interface and tweak type docs (modelcontextprotocol#1451)

Follow-up to modelcontextprotocol#955.

This factors out an `Icons` interface to centralize the implementation
and documentation of the various `icons` properties.

This also cleans up the type docs a bit, including removing reference to
a nonexistent `kind` property from an earlier revision of the SEP.

Author: jonathanhefner