Update all links to refer to the HTTPS endpoint

[wouterbeek]

The links from the project README link to the HTTP version of the website and endpoint. Only later I discovered that the website and endpoint are also available over HTTPS.

HTTPS has many benefits over HTTP, especially w.r.t. the SPARQL endpoint. For security reasons, HTTPS applications are not allowed to send requests to the HTTP endpoint.

Suggestion: update the web site and documentation to always point to HTTPS version of the web site and endpoint.