[docs] consider generating long-form script docs from embedded ldoc sections

[pq]

rather than having script docs split into two places:

• scripts/<author>/<script>.lua (for the menu docs)
• docs/<author>/script/index.md (for the long form ones)

we might consider pulling the long-form script docs into the script source itself and parsing it out with a custom ldoc filter.

while we're at it, we could define and process some custom tags to do things like build app indexes. things we might want to index include:

• grid-requiring apps
• midi-supporting apps
• arc-friendly ones
• utilities
• samples and studies
• templates
• experiments?

@tehn: i think you had other ideas?