Docs: YouTube Provider

Documentation for the YouTube provider. The provider ships in both platform packages (`packages/media/src/dom/youtube/`, `packages/html/src/media/youtube-video/`, `packages/react/src/media/youtube-video/`) and has sandbox templates (`html-youtube-video`, `react-youtube-video`), but it is undiscoverable from the site: - No reference page under `site/src/content/docs/reference/` (compare `vimeo`, which is also missing, and `hlsjs-video.mdx` / `dash-video.mdx` / `mux-video.mdx`, which exist). - `youtube` is absent from the `Renderer` union in `site/src/utils/installation/types.ts`, so it never appears in the installation page's "Choose your media source type" picker. This matters for Plyr migrators in particular — YouTube and Vimeo embeds are a large part of why people reach for Plyr. ## Prior art - #1790 — `Docs: YouTube Provider`'s sibling for the Vimeo provider, same shape. - `site/src/content/docs/reference/hlsjs-video.mdx` — existing provider reference page to model. - `.agents/skills/write-api-reference` — the API-reference workflow. Part of #1434.