docs: restructure README into a front-door + docs/ pages and embed it in rustdoc (#225)

* docs: restructure README into front-door with docs/ pages for config, cookies, editors, and scripting

* chore: drop dead config fields and fix flake feature

* chore(deps): bump idna, ring, rand to clear security advisories

* chore: drop unmaintained CHANGELOG and placeholder SECURITY in favor of GitHub Releases

* chore: embed README into crate-level rustdoc and bump yanked futures-util

Author: clearloop