[Cache] ELI5: Simplify cache-rules docs and fix critical bugs

[Oxyjun]

Summary

Apply ELI5 analysis to 6 files in docs/cache/how-to/cache-rules/ (terraform-example.mdx skipped — correct as-is).

Critical fixes

• settings.mdx: 3 invalid JSON examples with missing commas — would fail if copied by users
• create-api.mdx: Wrong API link — pointed to Update ruleset endpoint instead of Update rule endpoint
• create-dashboard.mdx: Skipped step number (jumped from 6 to 8)

Other changes

• index.mdx: Tighten run-on sentence, add Oxford comma
• order.mdx: Fix inconsistent Cache Rules capitalization (6 occurrences), remove future tense
• settings.mdx: Fix misplaced hyphen (cache control-header to cache-control header), fix first person (our to the, we recommend to Cloudflare recommends), fix wordy phrasing
• page-rules-migration.mdx: Add explanation of what Cache Everything means for migrating users, fix redundant note label
• create-dashboard.mdx: Replace cloudflare.com with example.com per style guide

[github-actions]

This pull request requires reviews from CODEOWNERS as it changes files that match the following patterns:

Pattern	Owners
/src/content/docs/cache/	@angelampcosta, @dcpena, @cloudflare/pcx-technical-writing, @cloudflare/product-owners

[github-actions]

Preview URL: https://9bab0054.preview.developers.cloudflare.com
Preview Branch URL: https://jun-cache-how-to-cache-rules-eli5.preview.developers.cloudflare.com

Files with changes (up to 15)

Original Link	Updated Link
https://developers.cloudflare.com/cache/how-to/cache-rules/settings/	https://jun-cache-how-to-cache-rules-eli5.preview.developers.cloudflare.com/cache/how-to/cache-rules/settings/
https://developers.cloudflare.com/cache/how-to/cache-rules/create-dashboard/	https://jun-cache-how-to-cache-rules-eli5.preview.developers.cloudflare.com/cache/how-to/cache-rules/create-dashboard/
https://developers.cloudflare.com/cache/how-to/cache-rules/order/	https://jun-cache-how-to-cache-rules-eli5.preview.developers.cloudflare.com/cache/how-to/cache-rules/order/
https://developers.cloudflare.com/cache/how-to/cache-rules/page-rules-migration/	https://jun-cache-how-to-cache-rules-eli5.preview.developers.cloudflare.com/cache/how-to/cache-rules/page-rules-migration/
https://developers.cloudflare.com/cache/how-to/cache-rules/	https://jun-cache-how-to-cache-rules-eli5.preview.developers.cloudflare.com/cache/how-to/cache-rules/
https://developers.cloudflare.com/cache/how-to/cache-rules/create-api/	https://jun-cache-how-to-cache-rules-eli5.preview.developers.cloudflare.com/cache/how-to/cache-rules/create-api/