docs: add Jun 18 changelog entry for Community Notes pricing fix

[mintlify]

Summary

Adds this week's changelog entry covering the Community Notes endpoint pricing correction.

Changes

• New Jun 18, 2026 entry in changelog.mdx noting that Community Notes reads are billed at $0.005 per resource under the standard Note: Read rate, with links to the Community Notes intro and pricing page.

Context

Reflects the merge to main (fix community note endpoint pricing) since the previous changelog entry on Jun 11, 2026.

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