feat: cache getCollection() calls in production

[bholmesdev]

Changes

Follows up on a performance bottleneck we found while migrating the Astro docs to content collections. @delucis noted that repeat calls to getCollection('docs') slowed production builds, and centralizing to a single, top-level getCollection() call resolved the issue. This PR introduces a production-only cache to mirror this experience.

Testing

• Created a branch on the docs repo to measure production build changes in CI: [DRAFT] getCollection perf improvements when called multiple times docs#2816 Found a ~10% improvement
• Built three times locally with caching and three times without. Noticed the same ~10% improvement
• Tracked memory usage building locally with and without caching. Each peeked at roughly the same 4.30 GB figure.

Docs

N/A

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