It's just simple relative links. If you're in /ipfs/QmPage/wiki/Page.html, then a link to './OtherPage.html' will result in /ipfs/QmPage/wiki/OtherPage.html.
Bingo. Then you have IPNS, which uses a public/private key system to broadcast updates to a mutable hash value. So you can take the IPNS public key for so-and-so's Wikipedia mirror, query the network for the latest hash signed by the corresponding private key, and use that hash to fetch content from IPFS.
