Document libraries & sample datasets

[Fil]

#294 created a placeholder page for each of the recommended libraries. Tracking here:

• Arquero (document arquero #321)
• Apache Arrow
• D3
• DOT (Graphviz)
• DuckDB
• Hypertext Literal
• Leaflet
• Lodash
• Mermaid
• Observable Inputs
• Observable Plot
• SQLite
• TeX
• TopoJSON

make it clear that these are just the "recommended" libraries, but that you can load any other library from npm — and that it will probably work, since it's just JavaScript. We don't what this list to indicate "boundaries", but rather "foundation".

Document sample datasets (#294 (comment)).

[Fil]

no need to document sample datasets since we'll remove them #373