docs: enhance rsbuild.context with access methods

[chenjiahan]

Summary

Expanded the description of rsbuild.context to include two methods of accessing the property (via the context property of the Rsbuild instance and via the api.context in plugins) and added code examples.

Checklist

• Tests updated (or not required).
• Documentation updated (or not required).

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Lighthouse	1 paths audited Performance: 74 (🔴 down 3 from production) Accessibility: 97 (no change from production) Best Practices: 92 (no change from production) SEO: 100 (no change from production) PWA: 60 (no change from production) View the detailed breakdown and full score reports

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