Show user friendly error message when API requests are sent to the front-end

### Current Behavior:
A lot of new Dependency-Track users accidentally try to send API requests (like uploading an SBOM) directly to the front-end container, not knowing the front-end and API server are deployed separately. The nginx front-end then returns a 405 - Method not allowed response, but this doesn't provide the user with enough information to find out what the problem is.

### Proposed Behavior:
Override the default nginx 405 page with a custom page, providing the user with some guidance on how to solve the problem. Something along the lines of:
``` 405 - Method not allowed. This probably happened because an API request was sent to the Dependency-Track front-end application. The front-end and API server are deployed separately, so API calls should be made to the API container. See https://docs.dependencytrack.org/getting-started/distributions/ for more information. ```