Fix `squid` service configuration in signer guide for SPO

[jpraynaud]

Content

This PR includes some fix on the Squid service configuration of the Run a Mithril signer as an SPO:

• Add command to stop the previously installed Squid service before building from source.
• Add debug logs to the service.
• Add tip with a fix when a FATAL: /dev/null (13) Permission denied error occurs at service startup.

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 38s :stopwatch: +2s   959 tests ±0    959 :white_check_mark: ±0  0 :zzz: ±0  0 :x: ±0  1 053 runs  ±0  1 053 :white_check_mark: ±0  0 :zzz: ±0  0 :x: ±0 

Results for commit 7d789e86. ± Comparison against base commit 481b7e78.

:recycle: This comment has been updated with latest results.