[docs] initial documentation + add verbiage to devshell master
Splits the README into structured wiki-ready markdown under docs/: - docs/README.md — overview and index - docs/getting-started.md — building, running, reverse proxying - docs/configuration.md — all env vars - docs/build-features.md — cargo feature flags - docs/concepts/{vs-tap,relay}.md — stream behavior and relay/seeding concepts - docs/api/{filter,ingestion,crawler,firehose,pds,repos,database}.md — REST API - docs/xrpc/{atproto,hydrant,backlinks}.md — XRPC reference flake.nix: adds github:90-008/verbiage as an input and includes the verbiage package in the devshell so `verbiage docs/` can be used to develop docs locally. Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>