add modules workaround docs

[hieu2102]

This PR closes #213

Summary

Add docs/using-module.md describing a workaround to deploy a ValkeyCluster with module.

Features / Behaviour Changes

• Add docs/using-module.md
• Add link to docs/using-module.md from docs/quickstart.md

Implementation

Limitations

Testing

Checklist

Before submitting the PR make sure the following are checked:

• This Pull Request is related to one issue.
• Commit message explains what changed and why
• Tests are added or updated.
• Documentation files are updated.
• I have run pre-commit locally (pre-commit run --all-files or hooks on commit)

[greptile-apps]

Greptile Summary

Adds docs/using-modules.md, a workaround guide for deploying a ValkeyCluster with a custom module (valkey-json), and links to it from docs/quickstart.md.

• The guide walks through building a single-stage custom Valkey image with build tooling baked in, deploying a ValkeyCluster that references it, and verifying the module loaded.
• The ValkeyCluster YAML example passes --loadmodule /opt/valkey-json/build/src/libjson.so as a single args list entry, which Kubernetes delivers to the Valkey process as one argv element; Valkey's argument parser will not recognize it as a valid directive, so the module will not load.

Confidence Score: 4/5

Documentation-only PR; the args format bug in the ValkeyCluster example would cause readers to produce a non-functional deployment, but no operator code is changed.

The args list in the ValkeyCluster YAML passes --loadmodule /opt/valkey-json/build/src/libjson.so as a single string. Kubernetes delivers each list item as a discrete argv element, so Valkey's argument parser receives the whole --loadmodule /path string as one token and cannot interpret it as a loadmodule directive. Any reader following the guide as written will get a cluster where the module silently never loads.

docs/using-modules.md — the ValkeyCluster YAML example at lines 70-71 needs the args split into two separate list items.

Important Files Changed

Filename	Overview
docs/using-modules.md	New guide for module usage workaround; contains an args-format bug that would prevent the module from loading, and a missing registry push step.
docs/quickstart.md	Adds a correctly-referenced link to the new using-modules guide; no issues.

Flowchart

%%{init: {'theme': 'neutral'}}%%
flowchart TD
    A[Developer machine] -->|docker build| B[valkey-customized:1.0 local image]
    B -->|missing step: docker push to registry| C[Container Registry]
    C -->|image pull| D[Kubernetes Nodes]
    A -->|kubectl apply| E[ValkeyCluster CR]
    E -->|operator reconciles| F[ValkeyNode pods]
    F -->|strategic merge patch containers spec| G[server container Command: valkey-server /config/valkey.conf Args: --loadmodule /path/to/libjson.so as ONE string]
    G -->|single argv element not parsed correctly| H[Module fails to load]
    G2[Corrected args: two separate list items] -->|two separate argv elements| H2[Module loads successfully]

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_{Reviews (4): Last reviewed commit: "update wording" | Re-trigger Greptile}

[bjosv]

CI hit the problem in #215, I trigger a CI rerun since this is only docs

[bjosv]

Very nice!

[jdheyburn]

LGTM, thank you for raising! cc @stockholmux to see if this is what you had in mind from chatting on Friday

[stockholmux]

A couple of nits but I think this makes a lot of sense for where we are today.

[jdheyburn]

Thanks for raising this! I'll get this merged now, we can refine it if we need to.