Docs: Update API docs guidance on --data use

We should add guidance to the Documenting REST API resources page about when and how to use curl's --data-urlencode option for API examples.

Background

The --data-urlencode option is useful for handling more complex Markdown formatting and other scenarios where URL encoding is needed. It's already being used in several API documentation pages, but we lack consistent guidance on when to use it.

The --data-urlencode option is currently used in:

• Container registry API
• Project import and export API
• User SSH and GPG keys API

Proposed changes

1. Add a section about --data-urlencode usage around the existing "Post data using curl's --data" section
2. Consider consolidating the various "Post data using" sections for better organization and consistency

Related issues

• Add complex Markdown example to Project wikis API (!214808 - merged)