chore: add changeset and update docs for configurable PII types

[yjouini]

Closes #458, Closes #459

Follow-up to #470 (main feature PR), adds the changeset and updates docs/06-chrome-extension.md to document the new PII entity types and custom patterns configuration options available in the extension settings.

[hanneshapke]

Tiny code example on how to read the list of labels and unify it.

import json
from urllib.request import urlopen

url = 'https://raw.githubusercontent.com/dataiku/kiji-proxy/main/model/quantized/label_mappings.json'
with urlopen(url) as r:
    m = json.load(r)
labels = m['pii']['id2label'].values()
entities = {l.split('-', 1)[1] for l in labels if l.startswith(('B-', 'I-'))}
print(sorted(e.title() for e in entities))

[yjouini]

Superseded — the backend changes landed via #509/#513, and the Chrome extension wiring is continued in #540 (rebased onto the new /api/pii/entities and /api/pii/regexes endpoints). Closing this.