Infra: generate machine-readable PEP index

[hugovk]

PEP 0 is a human-readable PEP index.

It would be useful to create a machine-readable version for, well, machines to read and process. This PR creates a peps.json of the key fields.

Preview:

[
{
"number": 1,
"title": "PEP Purpose and Guidelines",
"authors": "Warsaw, Hylton, Goodger, Coghlan",
"status": "Active",
"type": "Process",
"url": "https://peps.python.org/pep-0001/"
},
{
"number": 2,
"title": "Procedure for Adding New Modules",
"authors": "Faassen",
"status": "Superseded",
"type": "Process",
"url": "https://peps.python.org/pep-0002/"
},
{
"number": 3,
"title": "Guidelines for Handling Bug Reports",
"authors": "Hylton",
"status": "Withdrawn",
"type": "Process",
"url": "https://peps.python.org/pep-0003/"
},
...

https://pep-previews--2475.org.readthedocs.build/peps.json