Full documentation rework after the v2.1.0-beta dependency upgrade #827

Once the dependency upgrade / CTLie integration tracked in #815 and #826 is merged, the documentation needs a complete rework.

This is a full rewrite of docs/, not a series of targeted fixes — the upgrade changes enough of the user-facing API (differential geometry, the Flow call convention, extension arming, …) that patching existing pages page-by-page is not the right approach.

Depends on #826 (and #815) landing first.