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.

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docs/usage/configuration/config_documentation.md

@@ -3238,6 +3238,22 @@ Example configuration:
 ```yaml
 form_secret: <PRIVATE STRING>
 ```
+---
+### `form_secret_path`
+
+An alternative to [`form_secret`](#form_secret):
+allows the secret to be specified in an external file.
+
+The file should be a plain text file, containing only the secret.
+Synapse reads the secret from the given file once at startup.
+
+Example configuration:
+```yaml
+form_secret_path: /path/to/secrets/file
+```
+
+_Added in Synapse 1.125.0._
+
 ---
 ## Signing Keys
 Config options relating to signing keys