feat: local search, blog, PWA, CI, 404 page, and docs polish

[Aldiwildan77]

Local search:

• Add @easyops-cn/docusaurus-search-local with hashed index, keyed to the docs routeBasePath (/) so the search bar covers every page.

Blog / Changelog:

• Re-enable the blog preset as /changelog with RSS + Atom feeds.
• Ship one starter post announcing the docs refresh.
• Add "Changelog" link to the navbar.

PWA:

• Add @docusaurus/plugin-pwa with manifest.json, apple-touch-icon, and mask-icon. swRegister is off for now to avoid cache surprises.

CI:

• New ci.yml that runs yarn build on every PR — catches MDX errors and broken internal links before merge.
• New dependabot.yml to keep Docusaurus, Actions, and dev deps current.

Custom 404:

• Branded 404 page with three "go somewhere useful" tiles (Intro, Getting Started, Commands) and links to the FAQ and support server.

Open Graph image:

• New 1200×630 SVG at static/img/og-image.svg matching the land theme. OG and Twitter card metadata now point at it.

Favicon:

• Switch from the default Docusaurus favicon.ico to img/logo-mocha.svg.

Other polish:

• showLastUpdateAuthor enabled in docs preset.
• Copy-button CSS: visible border, gold hover accent, "copied" state.
• Admonitions on three pages: permissions info box in Getting Started, edit-window warning in Messages, Manage Webhooks note in Personalization.