Docs: add a headless / macOS rendering guide to guides/

The guides/ directory has a WindowsBuild.md but no equivalent for macOS, and nothing that covers running melt headless (no GUI, for batch/automated rendering). That's a real gap for anyone driving MLT from a server or a script.

I run headless melt on Apple Silicon daily as the render engine behind an automated video pipeline, and I've written down the parts that took time to work out. I'd be glad to contribute a tested guides/HeadlessRenderingMacOS.md covering:

• building/installing MLT on macOS (Homebrew + Apple Silicon notes)
• running melt with no display — the flags that actually matter for unattended/batch renders
• automated progress reporting (-progress2) for scripts and CI
• the one gotcha that bites people: use the pango producer for titles in a headless context rather than the drawtext/dynamictext path

I'd link it from Documentation.md alongside the Windows guide. Happy to open a PR — just wanted a nod on the scope and title first, and to confirm you'd want it under guides/ rather than developers/.

Thanks for MLT — it's load-bearing for what I build.

Start by reading guides/WindowsBuild.md for the existing guide structure and Documentation.md for the navigation link. Verify the documented macOS and Apple Silicon setup and headless melt workflow, including -progress2 and the pango title-producer note; done means adding guides/HeadlessRenderingMacOS.md and linking it alongside the Windows guide.