docs: update offchain subname guide and screenshots

[HAPPYS1NGH]

Summary

Update the Offchain subnames guide to match the current app, with seven screenshots and concise instructions for resolver setup, subname creation, record editing, and API keys. Preserve the opening image and show the complete resolver and API-key form screens.

Clarify that offchain subname creation, edits, and deletion require no blockchain transactions or gas fees; initial resolver setup does. Document owner-address indexing and restore the direct API-key link with a three-column scope comparison.

Remove the unwanted onchain architecture page and redirect its current and legacy URLs to the subnames overview.

Validation

• Mintlify build validation and broken-link checks passed.
• MDX accessibility checks and example TypeScript checks passed. Existing theme contrast recommendations remain.
• Local preview checked; screenshots load with descriptive alt text.
• Full npm run check passes, including content validation after removing the unwanted architecture page.

Updated screenshots

Resolver setup

Subname creation