Add Elasticsearch license activation guide

[tamalsaha]

Summary

• New guide under docs/guides/elasticsearch/license/ (_index.md, overview.md, license.md) modeled on the existing rotateauth guide's structure.
• Covers both ways to activate an Elastic subscription license: bring-your-own via a license.json-keyed Secret (spec.license.secretRef), and the built-in one-time trial (spec.license.trial).
• Covers the RotateLicense ElasticsearchOpsRequest for applying a license immediately instead of waiting for the next health-check pass.
• Explains the LicenseActive/LicenseExpiringSoon/LicenseSyncFailed status conditions and the self-healing continuous reconcile behavior.

Depends on

• Add per-CR Elasticsearch license activation support apimachinery#1912
• Add license activation methods to the Elasticsearch client db-client-go#268
• kubedb/elasticsearch#837

Marked draft since it documents a feature that isn't released yet; ready to un-draft once those land.

Test plan

• Verified all internal doc links resolve to existing pages
• Verified menu identifiers don't collide with existing ones and the parent identifier exists
• Docs site build/lint (CI)

🤖 Generated with Claude Code

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Visit the preview URL for this PR (updated for commit 2dfcdee):

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