Add form_secret_path config option (#18090)

I [was
told](https://github.com/element-hq/synapse/pull/17983#issuecomment-2593370897)
about another config option with a secret, so I got `form_secret` a
companion: `form_secret_path`

This PR makes NixOS and Kubernetes users a little bit happy. Includes
docs and tests.

### Pull Request Checklist

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* [x] [Code
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correct
(run the
[linters](https://element-hq.github.io/synapse/latest/development/contributing_guide.html#run-the-linters))