provide openapi document and link from homepage

Add an interactive openapi document at /ogcapi/openapi and link to it from /ogcapi

Links should be annotated as rel=service-desc && rel=service-doc, see example at https://demo.pygeoapi.io/master?f=json

- [x] on /ogcapi page, create a link to /ogcapi/openapi
- [x] /ogcapi/openapi?f=html should return the interactive api page
- [x] /ogcapi/openapi?f=json should return the json encoded OAS document
- [ ] add /ogcapi/openapi itself to the openapi document
- [ ] add /ogcapi/conformance to the openapi document