aspec: include doc build in ASpec again

Isabelle2020 doesn't allow sharing session directories between the document
session and non-document session. Instead of duplicating things, this commit
pulls the document build back into the ASpec session, but changes the build
such that the git revision is read directly from LaTeX, removing the
superfluous re-build for every git revision change (even when no relevant spec
file changed).

Signed-off-by: Gerwin Klein <gerwin.klein@data61.csiro.au>

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.