docs: document steps to make nix accept flakes

our current documentation says, that you can simply type `nix develop` which is not true (experimental feature flags).
We should document the actual dance that you have to do.

Find the current documentation that says to run `nix develop` and verify the experimental feature flags needed for flakes. Update the instructions to describe the actual setup and command sequence, with completion shown by a reader being able to use `nix develop` successfully.

Written by the indexing model from the issue text.