Finish ENA experiment-metadata templates for ~6 experiment types (incl. separate Nanopore/PacBio long-read templates)

Peter is producing ENA XML submission templates - mandatory + optional fields, plus blank and example XMLs - so users can start submitting experiment metadata to ENA now, using the existing `SRA.experiment.xsd` schema shared across ENA, NCBI, and DDBJ under INSDC. Draft XMLs were generated with AI assistance from the checklist spreadsheet + the XSD, and are now being manually corrected.

Covers roughly six experiment types: metabarcoding, exome sequencing, single-cell RNA-seq, bulk RNA-seq, and separate long-read templates for Nanopore and PacBio.