feat: add Discussion column linking each proposal to its forum thread

[dak-agent]

What

Adds a new Discussion column to both the MIPs and MRCs tables on mips.monad.xyz. Each row now has a Forum link that opens the proposal's discussion thread on the Monad forum.

How

• The link target is taken from each proposal's existing discussions-to: front-matter field (e.g. MIP-8 → https://forum.monad.xyz/t/mip-8-page-ified-storage-state/407), so every row points at its own thread.
• When a proposal has no discussions-to: set (currently only MIP-2), the cell falls back to the MIPs forum category: https://forum.monad.xyz/c/mips/8.
• Links open in a new tab (target="_blank" rel="noopener noreferrer") and are ignored by the row-click handler, so clicking Forum goes to the forum while clicking anywhere else in the row still opens the proposal.

Details

• New <th>Discussion</th> header + cell added to both the MIPs and MRCs tables.
• Rebalanced the fixed-layout column widths (@media (min-width: 700px)) to 6 columns (7 / 33 / 22 / 12 / 13 / 13 %) and bumped the mobile min-width to 1000px to accommodate the extra column.
• Added a small .discussion-link style (label + external-link glyph, kept on one line).

Verification

CI runs jekyll build on this PR, which validates the Liquid renders. Note that GitHub Pages deploy only runs on main, so the live site updates once this is merged.

[QEDK]

Does not render well on mid-size / mobile screens and looks pretty awful, furthermore discussion links are already present on MIP pages as icons and table rows, hence redundant, closing as not done,