Add spacy model download to PII sanitiser overview

[jbaross]

Jobs to be done

Follows from https://kongstrong.slack.com/archives/CDSTDSG9J/p1785991171667759

Definition of done

PII Sanitiser overview page and possibly how-tos should mention SpaCy model downloads so users in secure environments are aware that network calls to this repo are legitimate.

Check with dev team if any other plugins have similar behaviour.

Information

Size

• S (Small). Example: fixing a broken link, updating wording in an FAQ or paragraph