Checkers and better LaTeX templates

• Uses Microdown V2.7.1: introduces new elements to support Foliage and web generation. It improves the document checker. Now the following analyses are reported: duplicated anchors, references to unexisting anchors, unreferenced figures, missing figure files, and missing input files. https://github.com/pillar-markup/Microdown/releases/tag/v2.7.1
• Cleaned archetypes
• New documentation inside the bod-book template with the full syntax explained.
• Support for slides as header e.g. # a nice title Conversion of slides from pillar to Microdown
• Nicer support for extension e.g. {!comment|contents=sd thinks there is something fishy!} now can be expressed as {!comment|sd thinks there is something fishy!}`
• Two new archetypes are proposed: microdown-mooc and microdown-headerformat-mooc.
• Support for github actions with LaTeX.