base: broken link fix

* Fixes broken link to OpenAIRE FAQ.

Signed-off-by: Lars Holm Nielsen <lars.holm.nielsen@cern.ch>
base: API auth documentation update

* Updates OAuth 2.0 authentication documentation.

Signed-off-by: Lars Holm Nielsen <lars.holm.nielsen@cern.ch>
base: sign in/up with ORCID

* Enables sign in/up with ORCID.

Signed-off-by: Lars Holm Nielsen <lars.holm.nielsen@cern.ch>