chore: add page for tracking Borceux progress

[TOTBWF]

Description

Adds a page for tracking progress on Borceux. Somewhat surprisingly, we are already 34% of the way done!

Checklist

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• [x] I've read the contributing guidelines.
• [x] The imports of new modules have been sorted with support/sort-imports.hs (or nix run --experimental-features nix-command -f . sort-imports).
• [x] All new code blocks have "agda" as their language.

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[Lavenza]

Pull request preview

[plt-amy]

@TOTBWF can you add a mention of the Borceux page to the index? Under the HoTT and Elephant pages