Add RFC for data citation recommendations

[mfenner]

This pull request adds a high-level overview of the planned work to support the Data Citation Recommendations in invenioRDM. Most of the planned work is straightforward, but some topics need discussion, in particular:

• full implementation of givenName and familyName for personal authors. This is needed to properly expose author metadata in schemaOrg and bibtex, but is a breaking change.
• scope and detail of documentation for data citation that needs to be added
• whether or not to implement the optional recommendations content negotation and http link headers.

[mfenner]

Updated PR after call with @lnielsen on September 6.