[R2] Updating instructions regarding R2 APIs

[Oxyjun]

Summary

1. Updating instructions to match the new dashboard UI (which removes API {} dropdown, and instead has the Manage API tokens button directly from the Overview page).
2. Adds a banner on top of /r2/api/tokens/ page to clarify that these steps are different from generating an API token from My profile > API tokens ... route of creating it.

Screenshots (optional)

Documentation checklist

• Is there a changelog entry (guidelines)? If you don't add one for something awesome and new (however small) — how will our customers find out? Changelogs are automatically posted to RSS feeds, the Discord, and X.
• The documentation style guide has been adhered to.
• If a larger change - such as adding a new page- an issue has been opened in relation to any incorrect or out of date information that this PR fixes.
• Files which have changed name or location have been allocated redirects.

[github-actions]

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

Pattern	Owners
/src/content/docs/r2/	@oxyjun, @elithrar, @jonesphillip, @aninibread, @harshil1712, @cloudflare/workers-docs, @cloudflare/pcx-technical-writing

[github-actions]

Preview URL: https://e599c30c.preview.developers.cloudflare.com
Preview Branch URL: https://jun-r2-api-dash-audit.preview.developers.cloudflare.com

Files with changes (up to 15)

Original Link	Updated Link
https://developers.cloudflare.com/r2/api/tokens/	https://jun-r2-api-dash-audit.preview.developers.cloudflare.com/r2/api/tokens/
https://developers.cloudflare.com/r2/data-catalog/get-started/	https://jun-r2-api-dash-audit.preview.developers.cloudflare.com/r2/data-catalog/get-started/