Summary
- clarified Resources.init_session for low-level "isabelle process";
- clarified signature;
- clarified signature: theory imports could be from other sessions;
- proper html_symbols (amending 429afd0d1a79);
- clarified signature: master_dir is just Path.current;
- unused;
- HTML presentation in Isabelle/Scala, based on theory html exports from Isabelle/ML;
- clarified document;
- refer to HTML symbols via resources;
- moved lemmas from AFP to distribution