NfConfig

I suggested a while ago to add a tool for managing `nf-config` and `pkg-config`, but it was decided to only include `pkg-config`, not `nf-config`.

Unfortunately, when I removed `NfConfig` from the PR, I forgot to remove the documentation for this class, so atm the Fab documentation still states that `NfConfig` exists, and the code review didn't pick that up :(

We should clean up the documentation. I also want to raise the issue that I had to add `NfConfig` to MONC, so we now have two copies of the `NfConfig` tool (in lfric_core and monc), but I leave it to you to decide if you are happy with the code duplication, or want to change your decision and add `NfConfig` to Fab.

Search the Fab documentation for references to `NfConfig` and inspect the surrounding documentation entry. Remove or correct the stale documentation so Fab no longer claims that `NfConfig` exists. The possible duplication between lfric_core and monc, and whether Fab should include `NfConfig`, is a separate design decision to clarify before expanding the scope.

Written by the indexing model from the issue text.