Update schema link to 3.1

[bact]

• Add https://w3id.org/codemeta/3.1 link in JSON-LD page
• Update JSON examples to use 3.1 schema
• Also update the schema badge to use the same wording ("perma-id") as in README from codemeta/codemeta repo:
  https://github.com/codemeta/codemeta/blob/9458ead77eb8e5cc0df799bfd7f2a6d461525140/README.md?plain=1#L4