Implement fall-back function to give recommendations when Input Document is not in DB

[Joeran]

Let's say, a user calls https://api.mr-dlib.org/v1/documents/fes-bib-254998/related_documents but the document fes-bib-254998 does not exist in our DB/Solr Index, then it would be nice if we could return recommendations anyway. I see two options

a)

• We allow users to send a document's title in the URL, e.g. https://api.mr-dlib.org/v1/documents/fes-bib-254998;Gemeinsame%20Lösung%20anstreben/related_documents
• If we do not have the document in our database, we use the title to search Solr, and then return the search results as recommendations

b)

• If the input document is not in our DB, we return 0 recommendations
• Our user can then manually search for related documents by performing a classic search, e.g. https://api.mr-dlib.org/v1/search?q=Gemeinsame%20Lösung%20anstreben