[RFE] Headlamp demo website and branch previews

[sniok]

We can have a public stable read-only backend environment with sample cluster that has some realistic workload.
Then we can have a static deployments of the frontend that we can point to it.

With this we can achieve multiple nice things:

• Demo website (something like try.headlamp.dev/)
  People can browse and try the headlamp app without installing it.
  This can help bring more users; we can use it to demonstrate new features.
• Branch previews
  Have a separate frontend environment for each PR or branch. Can be very useful for a lot of reasons.

Here's how it could work:

Some drawbacks:

• only one version of the backend, can't do branch preview of backend changes
• we need to make sure backend is really read-only and secure (can be also be a good thing, make sure the backend is secure)
• more things to maintain