Docs: enhance runbook for aggregator metrics and signer registrations

[jpraynaud]

Content

This pull request includes updates to the runbooks for monitoring metrics and signer registration monitoring. The changes improve clarity and provide more detailed examples.

Documentation updates:

• docs/runbook/aggregator-metrics/README.md: Updated the description of the metrics_per_day view and provided a clearer example of how to display aggregated metric values by day.
• docs/runbook/registrations-monitoring/README.md: Renamed the section to "Signer registration monitoring" and updated the description of the signer_registration_summary view with a clearer example of how to display aggregated signer versions and total stake by epoch.

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)

Issue(s)

Relates to #2076

[github-actions]

Test Results

    4 files  ±0     51 suites  ±0   10m 30s ⏱️ -1s
1 417 tests ±0  1 417 ✅ ±0  0 💤 ±0  0 ❌ ±0 
1 628 runs  ±0  1 628 ✅ ±0  0 💤 ±0  0 ❌ ±0 

Results for commit 9810414. ± Comparison against base commit c56b384.

♻️ This comment has been updated with latest results.

[dlachaume]

LGTM 👍

[sfauvel]

LGTM