Publish theme to the npm registry as @docsy/theme

[chalin]

Publish Docsy to the npm registry as @docsy/theme, so that sites can install the theme (and its bundled npm dependencies) directly from the registry rather than from GitHub.

Planned scope, roughly in PR order:

• Make theme/ publishable: package metadata, files allowlist, bundled gen-favicons helper
• Docs: registry install as the lead install option
• Publish @docsy/theme (RC rehearsal, then at 0.16.0 tag time)
• Announce in the 0.16.0 release post