Update frontend config docs on readthedocs

[podliashanyk]

Documentation is

• [ ] incomplete
• [ ] incorrect or misleading
• [ ] outdated
• [x] missing

Where?

The documentation is missing at readthedocs

Expected documentation

I expected to find this:

Documentation on how to configure Argus frontend after the changes implemented in https://github.com/Uninett/Argus-frontend/pull/536

Actual documentation

The current feature/behaviour is not documented at all.