New home and embedding for the libre-graph-api (swagger-ui) #123

While migrating the dev docs to antora, the swagger-ui for the libre-graph-api is one element that needs to be coverd.

The current API definition is located in repo: https://github.com/owncloud/libre-graph-api which provides a subfolder that has been added to owncloud.dev (https://owncloud.dev/libre-graph-api). Because owncloud.dev is techincally dead and the migration is in progess, the libre-graph-api needs migration too.

One idea for the migration is, to use the Antora Swagger UI Extension from opendevise (who's maintainer is the head of the Antora development).

With the extension, we can use the collector extension to download the source from remote and handle it as it would be already saved in the attachment folder.

These are the locations in the ocis dev docs where libre-graph-api is mentioned:

grep -rl libre-graph-api

pages/apis/http/graph/spaces.adoc
pages/apis/http/graph/role.adoc
pages/apis/http/graph/permissions.adoc
pages/apis/http/graph/index.adoc
pages/ocis/adr/0022-sharing-and-space-management-api.adoc
pages/ocis/adr/0007-api-for-spaces.adoc
pages/ocis/adr/0023-index-and-store-metadata.adoc
pages/services/s-list/groups/index.adoc
pages/services/s-list/graph/index.adoc

This needs a lot of testing and checking, but if that works, it would not break the sourcing repo of the libre-graph-api...