Add Health Check category group

[andrepiske]

Adds the Health Check category group with a non-extensive list of projects. Addresses rubytoolbox/rubytoolbox#1090

The goal is to list gems that provide some form of web/API endpoint(s) that perform health check (or liveness/readiness checks) by a third party. This is not about gems that call those endpoints.

Before submitting your pull request:

• If you're referencing a gem by its GitHub repository (e.g. rails/rails), please verify
  that it is not packaged as a Ruby gem. (If it is packaged as a gem, please reference it
  by its gem name instead, e.g. rails.)
• Please make sure the projects are listed in case-insensitive alphabetical order
• Make sure the CI build passes, we validate your proposed changes in the test suite :)

[colszowka]

Thanks for contributing @andrepiske, please check out the 2 comments I have left, other than that good to go 👍

[colszowka]

This indentation is wrong and needs to be top level, this currently makes the build fail

[colszowka]

We should go with plural "Health Checks" here please, i.e. we have "Web App Frameworks"; "JSON Parsers" and so on, and I suppose the gems themselves might also be capable of doing multiple checks depending on the use case so plural makes sense there as well

[colszowka]

When in doubt, it would also be a nice to add a brief category description while we're at it, something like

Easing the setup of uptime health checks for your applications that can be used for monitoring and deployment automation

[andrepiske]

@colszowka hey I've made the changes you requested! Just took me a few months 🙄

[colszowka]

Thanks @andrepiske, only a few months later this is now reviewed and I'm merging this, thanks for your contribution!