aspec: reintroduce spec document version information

Including version information in the spec document is tricky, because
Isabelle will rebuild the session whenever it sees that session inputs
(including document sources) have changed. Since ASpec is close to the
root of our session hierarchy, frequently changing version information
causes excessive rebuilds during development.

This commit avoids excessive rebuilding by building the document (with
version information) in a separate ASpecDoc session. The ASpecDoc
session is identical to the previous version of the ASpec session, but
is not the parent of any other sessions. The ASpec session is used as
the basis for other sessions, but has document-only inputs removed, and
also has document builds disabled.