Reorganize toolchain

[boisgera]

Support for:

• short vs long content (reveal notes in slides ?)
• font awesome -> emoji
• automatic image insertion in slides ? image as backgroud (see https://salferrarello.com/full-screen-image-in-reveal-js/) ?
• more generally, use jupyter cells output ?
• hidden code (to be executed)
• target-dependent sections (via key-value assignments) ?
• style reveal with left-justified text by default ?

Backends:

• Slides (reveal),
• Jupyter notebooks,
• Starboard (Jupystar first ?)