[DOC] - How to add custom link to top bar

[asmacdo]

Preliminary Checks

• This issue is not a question, feature request, RFC, or anything other than a bug report. Please post those things in GitHub Discussions: https://github.com/nebari-dev/nebari/discussions

Summary

On this top bar, we'd like to add a custom link to user-facing documentation, ie how to use the hub, conda-store etc.

We are not using jhub apps which would be configured here

@viniciusdc suggested in the in the community meeting the first thing to try would be

jupyterhub: 
  overrides:
    hub: 
      services: 
         custom-doc: url
```
(services dict syntax may not be quite like that) 

[viniciusdc]

Hey @asmacdo since you are not using jhub-apps, this means that the navbar config comes directly from the nebari jupyter theme

Specifically the changes are done in here:
https://github.com/nebari-dev/nebari-jupyterhub-theme/blob/bca479b0bc912ada1947f1cd63961d4fb61c00f3/nebari_jupyterhub_theme/templates/page.html#L28-L37

Right now, this is hardcoded. I suggest opening a PR so that this is dynamic instead of being based on a new variable called services. Then in the nebari-config that would be placed under the themes section