Added new anti pattern blocking async code

[dhedgepeth]

📋 Description

Adds new section documenting an anti-pattern for blocking async code, something we are seeing more frequently.

✅ Contributor Checklist

I've followed the Umbraco Documentation Style Guide and can confirm that:

• Code blocks are correctly formatted.
• Sentences are short and clear (preferably under 25 words).
• Passive voice and first-person language (“we”, “I”) are avoided.
• Relevant pages are linked.
• All links work and point to the correct resources.
• Screenshots or diagrams are included if useful.
• Any code examples or instructions have been tested.
• Typos, broken links, and broken images are fixed.

Product & Version (if relevant)

CMS 13-17

📚 Helpful Resources

• 🧾 Umbraco Contribution Guidelines
• ✍️ Umbraco Documentation Style Guide

[sofietoft]

Thanks for the PR @dhedgepeth ! 🙌

Could you remove the changes to the v15 article from this PR? The version has been unpublished and removed, so that's why it's currently causing merge conflict.

Also, the vale linter is warning about the use of "relatively" and "significantly". Could you rewrite the sentences where these words are used?

Finally: Do you need a technical review on this as well?