bug: broken link on SUMMARY.md

[jcstein]

#172 attempted to fix a broken link to the homepage on sidebar, but it does not work as (/). See the workflow run error: https://github.com/celestiaorg/CIPs/actions/runs/10041673697/job/27750291351

The fix for this is to switch back the link to (./README.md) and wait for the https://cips.celestia.org domain to be live. Once it is live, it will work the same way that the README link works on the preview at https://cips.jcstein.dev