Add ability to add custom external links in side menu via options.

[TrueBurn]

Have the ability to add custom external links in the UI client's side menu: src/HealthChecks.UI/client/routes/Routes.tsx

It can be useful to be able to add links to external like ElasticSearch or company internal docs (confluence) etc. We use these links quite a bit on Spring Boot Admin and would be useful to have it for this C# solution as well.