feat(gateway): add AWS secret vault ref syntax

[STARRY-S]

Description

Add an info callout to the Gateway and KIC AWS Secrets Manager vault guides explaining how the reference syntax differs by secret type.

Fixes FTI-7221

Preview Links

WIP

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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Name	Link
🔨 Latest commit	7a790a0

[CLAassistant]

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[Copilot]

Pull request overview

Adds clarifying guidance to the AWS Secrets Manager vault how-tos (Gateway + KIC) so readers understand the correct vault:// reference syntax for different AWS secret value formats (plaintext vs JSON key/value), reducing confusion when referencing secrets in Kong.

Changes:

• Added an info callout describing how vault:// reference syntax differs for plaintext vs key/value (JSON) secrets.
• Documented the constraint that key/value secrets must be flat JSON with string values (no nesting / non-strings).

Reviewed changes

Copilot reviewed 2 out of 2 changed files in this pull request and generated no comments.

File	Description
app/_how-tos/kubernetes-ingress-controller/kic-vault-aws.md	Adds an info callout explaining plaintext vs JSON secret reference syntax for the KIC AWS vault how-to.
app/_how-tos/gateway/configure-aws-secrets-manager-as-a-vault-backend-with-vault-entity.md	Adds the same info callout for the Gateway AWS Secrets Manager vault how-to.