chore(deps): move the docs theme to @the-rabbit-hole/docs-theme on public npm

[Bugs5382]

What and why

This site consumes the docs theme under its old name, from a vendored tarball:

"@the-rabbit-hole-tech/docs-theme": "file:the-rabbit-hole-tech-docs-theme-0.1.0.tgz"

with an .npmrc pointing at the GitHub Packages registry:

@the-rabbit-hole-tech:registry=https://npm.pkg.github.com
//npm.pkg.github.com/:_authToken=${NODE_AUTH_TOKEN}

The theme is now published to public npm as @the-rabbit-hole/docs-theme, at
1.0.0. Public npm needs no registry configuration and no token, so the vendored
tarball, the registry line, and the auth token all become unnecessary. Consuming
a registry version also means this site picks up theme fixes instead of being
frozen at whatever 0.1.0 contained.

Work

• Depend on @the-rabbit-hole/docs-theme at ^1.0.0 and delete the vendored
  the-rabbit-hole-tech-docs-theme-0.1.0.tgz.
• Update the three import sites in docusaurus.config.ts: the
  recommendedThemeConfig import, the plugins entry, and the
  require.resolve of styles/custom.css.
• Delete .npmrc, and drop NODE_AUTH_TOKEN from whichever workflow supplies
  it for the docs build.
• Update the references in README.md and AGENTS.md.

Acceptance

• The site builds with no .npmrc and no npm token present.
• No reference to @the-rabbit-hole-tech/docs-theme or npm.pkg.github.com
  remains.