fix: redirect mis-cased /MIPS paths to /MIPs

[dak-agent]

Summary

The link to MIP-12 shared in this tweet from Keone is broken:

• Broken: https://mips.monad.xyz/MIPS/MIP-12 — uppercase S in MIPS
• Works: https://mips.monad.xyz/MIPs/MIP-12 — lowercase s in MIPs

The site is served by GitHub Pages, which resolves paths case-sensitively, so /MIPS/... returns a 404 while /MIPs/... resolves.

Fix

Adds a small redirect handler to 404.html. When a request 404s because its leading collection segment is mis-cased, it normalises the segment to the canonical directory casing and redirects, preserving the rest of the path, the query string, and the hash:

• /MIPS/* → /MIPs/*
• /MRCS/* → /MRCs/*

This covers MIP-12 and the entire /MIPS/ (and /MRCS/) namespace generically — current and future proposals — rather than fixing one link at a time. Because GitHub Pages serves only static files (no server-side redirect rules), the redirect runs client-side on the 404 page via location.replace, so it does not add a browser history entry. Requests that already use the canonical casing are untouched, and a corrected path that still doesn't exist falls through to the normal 404 (no redirect loop).

[dak-agent]

Requesting review from @QEDK and @p-shahi 🙏

[p-shahi]

Closing as this has been a discussed internally per @QEDK

"absorbed the cost of future changes switching from MIPS -> MIPs early on so a lot of it is retrospective fitting that is not required"