blog: add 3.18.0 release notes

[kayx23]

Summary

• add English and Chinese release notes for Apache APISIX 3.18.0
• document 14 breaking changes with explicit upgrade plans, substantive feature coverage, and bug fixes grouped by affected subsystem
• use the confirmed August 20, 2026 release date in both the file paths and page metadata
• reconcile the release notes with the reviewed changelog update in chore: release 3.18.0 apisix#13842

Release source

• Apache APISIX 3.18.0, published August 20, 2026
• chore: release 3.18.0 apisix#13842 head 9bce154a, which addresses the release-note categorization review

Validation

• yarn lint:frontmatter
• repository pre-commit Remark checks for both Markdown files
• JSON and YAML example parsing
• English and Chinese heading-structure parity
• 14 explicit upgrade plans in each language, matching all 14 changelog :warning: entries
• Chinese terminology and localization audit against current APISIX Chinese documentation
• staged diff whitespace check