Docs: add a headless / Apple Silicon macOS rendering guide (#9)

[Magnussmari]

Adds guides/HeadlessRenderingMacOS.md as discussed in #9: install on Apple Silicon, running melt headless via the avformat consumer, -progress2 for scripts and CI, and headless title rendering. Linked from developers/BuildTips.md under the existing macOS subsection, mirroring the Windows guide.

Thanks for the pointer to mlt#1069, it reshaped the guide. I tested it on this machine and the guide leads with qtext as you suggested (it is richer), with one macOS-specific wrinkle worth having on record: #1069 fixes the Linux DISPLAY abort, but on macOS Qt runs the cocoa platform, so qtext already renders headless with no env var at all. The guide therefore presents qtext as the default, QT_QPA_PLATFORM=offscreen as the portable form for cross-platform scripts and CI, and pango only as the fallback for daemon contexts with no window server (also the one text path left when a stock brew FFmpeg 8 has no drawtext filter).

Verified on melt 7.38.0 / FFmpeg 8.1.1 / Apple Silicon: valid 1080p renders with qtext both with and without the offscreen var (text confirmed via luma), pango headless with no window server, and brew FFmpeg 8's missing drawtext filter. Front matter and Liquid link validated against the existing guides; I could not run a full local Jekyll build (the github-pages toolchain does not install cleanly on the system Ruby here), so a CI or preview render is worth a glance.

On the maintenance worry you raised: I should say I do not contribute to a spread of projects. I only send work to tools I actually use and think I can be of service to, and MLT is one of those. I run headless melt daily as the render engine of an automated pipeline, so this documents my production setup. I have dated the guide and I am glad to re-verify it against new MLT releases and take any macOS-doc issues you want to route my way. If you would prefer more discoverability I can add a pointer from MltMelt.md or Documentation.md, but I kept the diff minimal to match the Windows guide's placement.

[ddennedy]

I am sorry, but I misunderstood what you asked in #9. I thought you were going to write about "building" MLT on macOS. You even linked to the new page from the BuildTips page. Howeverl, this is not about that. It is basically get it from homebrew plus this collection of tips for rendering headless. Seems more like personal notes you want to publish on the project web site. Change this to put the link on the "Miscellaneous" section of https://mltframework.org/docs/ and shorten the line: Headless Rendering. It has useful tips not just for macOS and putting "macOS" in the line will lead people to dismiss it, but there are many package repos for all OS that include MLT.