docs: improve discoverability of health checks how-to guide

[chapman-von-monty]

Analytics showed that the reference page for health checks received
significantly more traffic than the how-to guide "How to run services
reliably", despite the how-to guide being positioned second in the section.
The page title and navigation lacked mention of health checks, making it
difficult to discover for users searching for check configuration and
service auto-restart behavior.

This improves the discoverability of the guide by:

1. Renaming the page title to "How to run services reliably with health checks"
   and the table of contents entry to "Configure health checks to run services
   reliably".
2. Adding an upfront callout under ## Usage in reference/health-checks.md
   and making the link under See also explicit.
3. Adding a cross-reference in reference/service-auto-restart.md pointing to
   the health check restart guide.
4. Featuring the guide on the documentation homepage in docs/index.md.
5. Linking to the guide under Next steps in the tutorial getting started doc.

Fixes #864

[hpidcock]

@dwilding my bot has had an attempt at #864, can you please review it so I can either have it try again or improve it's SOUL.