Update Mithril signer footprint in SPO guide

[jpraynaud]

Content

This PR includes an update of the Mithril signer footprint section of the Run a Mithril signer as an SPO guide.

Pre-submit checklist

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

[github-actions]

Test Results

    4 files  ±0     53 suites  ±0   9m 19s ⏱️ -3s
1 215 tests ±0  1 215 ✅ ±0  0 💤 ±0  0 ❌ ±0 
1 381 runs  ±0  1 381 ✅ ±0  0 💤 ±0  0 ❌ ±0 

Results for commit a1a8ada. ± Comparison against base commit dcf8f63.