[emnarahmouni-exmergo]

Turns the Mintlify starter kit into the Exmergo docs site at docs.exmergo.com.

• docs.json: the "Exmergo" site settings (blue palette, navbar, footer,
  search, analytics). The product switcher has two entries: Exmergo (the home
  page) and Dex, whose menu and redirects are read from dex/navigation.json
  and dex/redirects.json with $ref.
• index.mdx: the home page. "Exmergo is the company focused on building the
  AI Stack for Your Data Stack", with a card to /dex.
• skill.md: tells an agent that finds the site where Dex's real skills come
  from.
• README.md, AGENTS.md, .mintignore: rewritten for this repository.
• Mintlify's starter pages, logos and favicon are removed.
• .github/workflows/validate.yml: runs Mintlify's validate,
  broken-links --check-anchors --check-redirects and a11y over the whole
  site on every PR and push. It also fails a PR that edits a product folder by
  hand, unless the PR has the product-sync label.
• dex/: the first copy of the Dex section, built by exmergo/dex. From now on,
  every Dex release replaces this folder automatically.

This PR has the product-sync label because it adds dex/.

Open decisions

• The LICENSE is still Mintlify's MIT template (copyright "Mintlify").
• There are no Exmergo logo files yet, so the site shows its name as text.