[Access] Document ctx.access for JWT validation

[kennyj42]

Summary

Updates the Access JWT validation guide to point Workers developers to ctx.access, which provides the authenticated request context without manual JWT parsing. Keeps the manual verification example for cases that require it and links to the documented limitations.

Fixes #33810

Documentation checklist

• The change adheres to the documentation style guide.
• If a larger change - such as adding a new page - an issue has been opened in relation to any incorrect or out of date information that this PR fixes.

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Original Link	Updated Link
https://developers.cloudflare.com/cloudflare-one/access-controls/applications/http-apps/authorization-cookie/validating-json/	https://kjohnson-document-workers-ctx-access.preview.developers.cloudflare.com/cloudflare-one/access-controls/applications/http-apps/authorization-cookie/validating-json/

[cloudflare-docs-bot]

@kennyj42 This draft pull request has had no activity for 3 days. It will close after 7 days of inactivity. Transition to ready for review if ready. A codeowner can comment /draft-never-stale to keep it open.