Workflows: update limits and added notes

[bruxodasilva]

Summary

Updated the following information regarding Workflows:

• add note referring that Workflows do not support Workers for Platforms
• updated creation rate and concurrency limits

Closes WOR-940 and WOR-729.

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• If a larger change - such as adding a new page- an issue has been opened in relation to any incorrect or out of date information that this PR fixes.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