feat(internet-access): add landing page replacing the overview (#299)

- create landing-page-internet-access.mdx (pageType: landing) covering all 30 guides
- wire it as the "Internet access" sidebar landing, drop the Overview entry
- delete the old overview.mdx
- repoint internal links (home tile, ftth-fix-access, verifier-stabilite-acces,
  voip/troubleshoot-local-network) to the new landing