Add example EnzymeML files into spec and website

[cthoyt]

hello! I'm sitting at the NFDI4Chem consortium meeting and just saw a presentation for EnzymeML

I think it's great that it's defering to ontology standards, but I wasn't actually able to find some example files that are using the standard. It would be great if you could add some for interested onlookers like me

One of the things I would love to collaborate on is using the Bioregistry (https://bioregistry.io, https://github.com/biopragmatics/bioregistry) to help validate cross-references to external entities, e.g. to reactions. For example, it can say:

1. 'hey, we see you're referencing reactions from Rhea, that's great'
2. "hey, we see you're making references, but in a non-standard way. here's a fix"
3. "we're unaware of the kinds of references you're making, do you want to register the namespace/identifier space on the Bioregistry"?

@torogi94 Niels said I should chat with you!