fix(docs): Better trailing slash omission (#23304)

Partial reversion of #23286.

SWA's implementation of `"trailingSlash": "never"` is currently flawed.
The resulting redirect ends up exposing the underlying SWA URL of the
site, rather than our configured front door URL. E.g.
`https://fluidframework.com/docs/concepts/signals/` ends up redirecting
to
`https://salmon-sand-0b7fa7c1e.1.azurestaticapps.net/docs/concepts/signals`.
- See Azure/static-web-apps#1036

This PR removes that configuration flag, and instead leverages
Docusaurus's
[trailingSlash](https://docusaurus.io/docs/api/docusaurus-config#trailingSlash)
configuration option to not emit trailing slashes.

This required updating a handful of relative URL links to use file path
links. The guidance documentation in the README has been updated to
offer new guidance around relative links in light of new learnings.

[AB#25895](https://dev.azure.com/fluidframework/235294da-091d-4c29-84fc-cdfc3d90890b/_workitems/edit/25895)

docs/static/staticwebapp.config.json

@@ -14,6 +14,5 @@
 			"route": "/404",
 			"statusCode": 404
 		}
-	],
-	"trailingSlash": "never"
+	]
 }