explicit docs template dir

[turbomam]

Partial solution for [#227, #226]

• [x] new doc generation rule in project.Makefile that uses an explicit --template-directory
• [x] doc_templates/slot.md.jinja2 edited so that slot example values are displayed

[turbomam]

Sorry, I committed the autogenerated project/ files

[matentzn]

Thank you @turbomam!

The jinja 2 files - shouldnt they be supplied by linkml? what happens if these need to be updated?