Streamline README for end users, move technical docs to docs/

README now focuses on download/install, features, and usage.
Build instructions moved to docs/BUILDING.md. Custom filter
schema was already in docs/FILTER_SCHEMA.md. Updated feature
list with subtitles, batch queue, hardware encoding, etc.

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

Author: StuartCameronCode