Add customizable 'About' pages

[sbliven]

Is your feature request related to a problem? Please describe.
Currently there are no links to any documentation from the landing page. Although this is the public entrypoint for scicat and thus receives the most naive users, there is no way to provide any help information. We would like to add some way to link the following information:

• Contact page for support
• About page summarizing the service
• Links to our privacy policy/terms of service

Describe the solution you'd like
The challenge is making these pages customizable. I can think of two solutions:

1. Allow custom links to be added to the header or footer. The content would have to be served elsewhere.
2. Allow handlebars templates to be specified for additional pages. These would be applied statically during site generation to generate pages.

Additional context
This feature was requested by external users who had trouble finding our support contact info after encountering issues. Fortunately we were eventually forwarded the complaint through another source.

[minottic]

closed? @omkar-ethz

[omkar-ethz]

closed? @omkar-ethz

We only added a support email in #396, but no (customizable) About page with documentation etc as the issue mentions. so it is only partially closed.