docs(k8s-install): PLTF-1247 name Valkey as the bundled cache

[aivong-openhands]

Why

The chart's bundled cache is moving from Redis to the official Valkey chart. Valkey speaks the Redis protocol, so nothing about the install procedure changes: the redis secret and its redis-password key stay exactly as documented, because the chart deliberately keeps that name so existing installs need no new secret and can revert without re-deriving a credential. What changes is the component's name in the architecture table, the embedded-services sentence in Step 3, and the diagram label — each introducing Valkey as a Redis-compatible cache rather than assuming the reader knows it.

Two mentions are deliberately left as Redis: the secret name and key, which do not change, and the "integrate with existing infrastructure" line, which refers to a cache the reader already runs rather than to what the chart deploys.

The diagram label was edited in the SVG directly, as this repo holds only the rendered file and no diagram source. Whoever owns that source should apply the same change there, or the next regeneration will bring the old label back.

Validation

• Audited every cache mention on these pages — three describe what the chart deploys and are updated; the two that remain are the unchanged secret name and key, and the reference to the reader's own infrastructure.
• Reparsed the edited SVG — well-formed, 14 text nodes, and the only cache label present is now Valkey. The label uses text-anchor:middle, so it re-centres without touching geometry.

This PR was drafted by an AI agent on behalf of the user.

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[enyst]

I'm an AI agent based on Opus 5.5, helping Engel Nyst (@enyst) with project work.

Heads-up before this merges: the chart still deploys Redis by default. Valkey is opt-in (valkey.enabled: false, and Redis wins when both are on; see values.yaml#L865-L891). Making Valkey the bundled cache is OpenHands/OpenHands-Cloud#1008, which is still open. Merged now, these docs would tell operators that Valkey runs embedded when it doesn't.

Suggest holding this until #1008 lands, then rebasing. The "PostgreSQL and Redis run embedded" sentence is unchanged on main, so the rebase should be small.