docs: add Homebrew tap to installation page, replace COMING SOON

[vinceblock99]

Summary

Updates the Installation page to surface the new Homebrew tap:

• New macOS — Homebrew (Recommended) section, placed above the hosted installer.
• Hosted Installer retitled to "Linux, CI, no Homebrew" — still the cross-platform default.
• Distribution Packages section: replaces "COMING SOON" with the Homebrew tap entry and notes upcoming apt/dnf/pacman/Scoop/winget in the roadmap.
• Bumped `ATOMIC_VERSION=0.5.1` example to `0.6.0`.
• Intro line now covers Homebrew-on-Linux users (per review).
• Explicit anchor id `{#homebrew}` on the heading — em-dash slugify rules vary across renderers, explicit id is the safe default.

Three install layers stay clear: Homebrew → hosted installer → manual tarball → source.

Companion PR

Same content lands in the atomic README: atomicdotdev/atomic#58.

Test plan

• All command snippets verified against the working tap (locally installed via `brew install atomicdotdev/tap/atomic`)
• Local Docusaurus build (`npm run build`) succeeded with no broken-link errors
• Verified generated HTML: heading emits `id="homebrew"` and the internal link emits `href="#homebrew"` — anchor resolves
• Reviewer sanity check on copy / ordering