feat: revamp design

[QEDK]

• uses Monad branding (fonts, logo, colorways)
• adds a 404 page
• adds a status filter
• all other features preserved from previous version

[Copilot]

Pull request overview

This PR modernizes the MIPs site UI to match Monad branding, consolidates proposal rendering into a single layout, and upgrades the index experience with unified search + multi-select status filtering while preserving existing proposal content behavior.

Changes:

• Redesign index and proposal pages with new design-system-driven styling, header/footer, and updated branding assets.
• Add new shared client-side behaviors (assets/js/site.js) and move index-specific interactions into assets/js/index.js (tabs, search, status filter).
• Introduce a dedicated 404 page and consolidate MIP/MRC layouts into a single _layouts/proposal.html via _config.yml defaults.

Reviewed changes

Copilot reviewed 14 out of 20 changed files in this pull request and generated 2 comments.

Show a summary per file

File	Description
index.md	Rebuilds the homepage markup; delegates ledger rendering to an include; loads new index JS.
assets/js/site.js	New shared JS for syntax highlighting assist, link rewriting, author/linkification, tooltips, ToC/progress, copy actions, and prefetch.
assets/js/index.js	New index-only JS implementing tabs, unified search, and multi-select status filtering.
assets/ds/colors_and_type.css	Adds Monad design tokens and self-hosted font-face definitions.
assets/ds/buttons.css	Adds button styling used by the new 404 and potentially other CTA surfaces.
assets/css/site.scss	Replaces inline/legacy styling with a full site stylesheet (header, index, proposal detail, ToC, tooltips, 404).
assets/brand/logo-full-default.svg	Adds updated Monad logo asset for the new header.
404.html	Adds branded “Not found” page with CTA buttons back to index/repo.
.gitattributes	Marks .woff2 as binary to avoid EOL normalization issues.
_layouts/proposal.html	New unified proposal layout for both MIPs and MRCs (meta rail, citation, actions).
_layouts/mrc.html	Removes legacy MRC layout (replaced by proposal).
_layouts/mip.html	Removes legacy MIP layout (replaced by proposal).
_layouts/default.html	Adds new global header; moves styles to external CSS; loads shared JS; updates title kind detection to use page.category.
_includes/ledger.html	New shared ledger table include used by the index.
_config.yml	Updates defaults to use layout: "proposal" and enables compressed Sass output.

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