feat(event-gateway): mask field policy guide

[jakubdyszkiewicz]

Description

https://konghq.atlassian.net/browse/EVG-356

A new policy for masking fields.

To test start ewg this way

export KONNECT_DOMAIN=konghq.tech
export KONNECT_REGION=us
export KONNECT_EVENT_GATEWAY_MIN_VERSION=1.3

curl -Ls https://get.konghq.com/event-gateway | bash -s -- \
  -k $KONNECT_TOKEN \
  -N kafka_event_gateway \
  -n jd-mask-test \
  -i kong-event-gateway-dev \
  -t main-d96d6a8

Preview Links

Checklist

• Tested how-to docs. If not, note why here.
• All pages contain metadata.
• Any new docs link to existing docs.
• All autogenerated instructions render correctly (API, decK, Konnect, Kong Manager).
• Style guide (capitalized gateway entities, placeholder URLs) implemented correctly.
• Every page has a description entry in frontmatter.
• Add new pages to the product documentation index (if applicable).

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