Relax fixed path requirements and clarify required endpoints

[jerstlouis]

To fully support the use cases of static deployment on cloud object storage, we are proposing to require only the "follow the link" approach.

This would avoid unnecessary divergence with a GeoDataCube API.

Also clarify which endpoints are necessary (landing page, collections, collection description, coverage?).

cc. @m-mohr

[jerstlouis]

We will relax the fixed path for /coverage, since this is what would be impossible to move for static use cases, whereas the metadata can easily be provided locally at an arbitrary path.

We will also relax the requirement for /schema since that is not specified in Features - Part 5 or Common equivalent.

For now we will keep the dependency on Common - Part 2 and associated fixed paths (landing page, collections, collection description).