Add further sources

[xlrx]

• SemanticScholar API: https://api.semanticscholar.org/v1/paper/10.1038/nrn3241
• PubMed (https://www.ncbi.nlm.nih.gov/pmc/tools/developers/)
• OpenCitations
• (OpenAIRE)

[lukasjesche]

check these

• https://libraries.mit.edu/scholarly/publishing/apis-for-scholarly-resources/
• https://guides.lib.berkeley.edu/information-studies/apis