Improve identifiers page

[cthoyt]

identifiers page (https://knowledgebase.nfdi4chem.de/staging/knowledge_base/docs/identifiers/) could have more generic text about identifiers, that goes beyond PIDs as a popularity contest

• add in some discussion about Semantic Farm on that page and using good identifier practices, esp Identifiers for the 21st Century
• reference Semantic Farm for finding different identifier schemes for annotating data / for registering databases, ontologies, and new resources (use some of the specific ones from NFDI4chem as an example)
• Reference Semantic Farm collections / NFDI Section Metadata recommendations Add recommendation on consortium ontology collections nfdi-de/section-metadata-wg-onto#81