Improve docs coverage and validation

[rampage644]

This expands the docs from a Docker-only and Lambda-focused outline into a fuller release-readiness set with self-hosted local binary coverage, a first-class Snowplow workflow, deeper Lambda/dbt guidance, and clearer chooser/support pages. It also strengthens docs validation with Astro checks, repo-safe smoke checks, and a full docs build so key paths and repeated constants are less likely to drift.