[docs] add missing pattern entries for account_based and key_request

[Danny-Devs]

Description

ExamplePatterns.md documents 5 of the 7 access patterns in move/patterns/sources/, but account_based and key_request are missing. This adds both entries following the existing style (use-case-first, single paragraph, link to Move source).

• account_based — stateless, address-derived encryption for messaging and notifications
• key_request — two-step pattern that separates policy evaluation from the Seal approval step

Placement: account_based is inserted between Private data and Allowlist to follow a complexity gradient (single owner → specific address → group). key_request is placed last because it's a meta-pattern that wraps other patterns rather than defining its own access policy.

Also fixes "ay" → "at" in CONTRIBUTING.md line 27.

Written with the help of Claude Code.

Test plan

• Verified both new sections render correctly with mkdocs serve
• Cross-checked descriptions against Move source code
• Docs only, no code changes

[Danny-Devs]

Updated to target the new docs/content/ExamplePatterns.mdx after the mkDocs → Docusaurus migration in #468. Same two entries, matched to the new file format.