Add television input-mode and eARC troubleshooting notes

[cortezbuilds]

The HDMI audio guide explains output-device selection but does not cover a television's per-port enhanced-input mode or forwarding audio through the TV to a soundbar/receiver. Add two short troubleshooting notes so readers can check those settings before assuming that Linux is limiting the connection.

The input-mode note is vendor-neutral, links Samsung's Input Signal Plus documentation as an example, and does not promise a particular refresh rate. The audio note distinguishes PCM channel selection from encoded pass-through and Dolby Atmos support.

Validation: the full Astro build passed (145 pages), and the new section was inspected in the browser with its headings, text, table-of-contents links, and manufacturer link rendered correctly. The patch applies cleanly without added whitespace errors. The repository's Prettier configuration intentionally ignores MDX; both the original and revised article also passed an independent MDX parse check. No unrelated files or formatting changes are included.

This documentation change was prepared with AI assistance.