Root document contains broken links for the conformance and service-desc documents

[jfbourgon]

Describe the bug
Since the implementation of PR #792, when starting Franklin in serve mode with an --api-path parameter, the root document contains broken links for the conformance and service-desc documents.

For instance, here are the suggested broken links when starting Franklin with --api-path=api. All the other suggested links (search, collections, root) are valid.

"links": [
{
"href": "https://<hostname>/api/open-api/spec.yaml",
"rel": "service-desc",
"type": "application/vnd.oai.openapi+json;version=3.0",
"title": "Open API 3 Documentation"
},
{
"href": "https://<hostname>/api/conformance",
"rel": "conformance",
"type": "application/json",
"title": "Conformance"
},
]

Expected behavior
The expected behavior would be that these links would be reachable at the suggested locations.

A second option could be that these service documents could be advertised using their current location (outside of the suggested api-path prefix). Which are currently the following:
service-desc: https://<hostname>/open-api/spec.yaml
conformance: https://<hostname>/conformance

The first option would be the preferable one since all documents related to Franklin STAC API could be associated to a common path prefix but the second option could also be acceptable.