chore(aisix): add Artifact Hub package metadata

[Yilialinn]

What this PR does

Prepares the existing aisix Helm chart for an Artifact Hub listing by adding
package discovery metadata:

• the ai-machine-learning category and AISIX AI Gateway alternative name;
• the AISIX 1.2.0 container image for package scanning;
• named source, documentation, homepage, and support links;
• a chart-level changelog entry.

The chart version is bumped to 1.2.1 while appVersion remains 1.2.0 because
this is a chart-only metadata change.

Scope and behavior

This remains a data-plane chart that requires an AISIX control plane and mTLS
certificate. The change does not make it a standalone AISIX installation or
change workload behavior. Rendered manifests differ only in the standard
helm.sh/chart version labels produced by the chart version bump.

Artifact Hub repository registration is a separate post-merge step.

Verification

• helm lint charts/aisix -f charts/aisix/ci/default-values.yaml
• helm template with the documented control-plane and certificate values
• helm package charts/aisix
• verified the referenced AISIX 1.2.0 image and public links

[coderabbitai]

Important

• 🔍 Trigger review

This repository does not receive automatic reviews because it has fewer than 10 stars.

⚙️ Run configuration

Configuration used: Organization UI

Review profile: CHILL

Plan: Advanced

Run ID: b0e96f9f-0a8e-4822-a386-e26a06cd4208

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_{Comment @coderabbitai help to get the list of available commands.}

[Yilialinn]

Closing this for now because Artifact Hub registers Helm repositories rather than individual charts, and we are not currently planning to register the full charts.api7.ai repository, which contains several API7 charts beyond AISIX. We can revisit this if we decide to register that repository or publish AISIX through a dedicated Helm or OCI endpoint. Thanks.