Add `Mithril Protocol Insights` dashboard link on website

[jpraynaud]

Content

This PR includes a new link in the footer of the website to the Mithril Protocol Insights dashboard.

Pre-submit checklist

• Branch
  • [x] Commit sequence broadly makes sense
  • [x] Key commits have useful messages
• PR
  • [x] No clippy warnings in the CI
  • [x] Self-reviewed the diff
  • [x] Useful pull request description
  • [x] Reviewer requested
• Documentation
  • [x] Update documentation website (if relevant)

[github-actions[bot]]

Test Results

    3 files  ±0     43 suites  ±0   8m 39s :stopwatch: -1s   971 tests ±0    971 :white_check_mark: ±0  0 :zzz: ±0  0 :x: ±0  1 065 runs  ±0  1 065 :white_check_mark: ±0  0 :zzz: ±0  0 :x: ±0 

Results for commit 3923d58b. ± Comparison against base commit 1ef4fb76.