Display a tooltip when hovering over an EIP/ERC link

[SamWilsn]

I'd like to see a tooltip that appears when hovering over EIP links (eg. [EIP-20](./00020.md)). It should contain at least the title and description, and can include other optional information.

[ritorhymes]

Implemented this as a hover/focus preview popover for proposal links. It shows structured proposal metadata: number, title, description, status, type, and category.

PRs:

• Preprocessor metadata feed: Generate Proposal Metadata JSON preprocessor#34
• Theme preview UI: Add Proposal Link Popovers #13