Drop PR previews and finish the remaining documentation chapters

[mikelangmayr]

• Remove the PR preview deployment: the workflow drops two third-party actions, the cleanup job and the pull-requests: write permission, leaving one job. Rendered output for a pull request is the uploaded artifact.
• Publishing no longer needs keep_files, so a merge to main now fully replaces the site and stale pages are cleaned up instead of lingering
• Getting started now walks through config/frame_outputs_test/frame_outputs_test.cfg, the config CI drives end to end. The previous walkthrough used config/demo/demo.cfg, which enables no frame outputs, so the promised FITS file was never written.
• FITS chapter documents the real naming scheme (FITS_BASENAME plus a zero-padded frame number, with a collision suffix) and corrects the claim that fitsnaming, imnum and fitsname control it; those commands no longer exist
• Controller commands chapter records that native on an ARC build returns DONE while doing nothing, and that controller_cmd is Archon-only, so the ARC passthrough does not currently work
• Development chapter covers adding a command or key, and warns that only hispec_tracking_camera is ever built in CI