docs: add best practices example for #[derive(Insertable)]

[Mahi-korrapati]

This PR adds a new section in the documentation under guide_drafts/best-practices.md
describing how to avoid namespace conflicts when using #[derive(Insertable)].

✅ Includes:

• Correct example using a single table DSL (use crate::schema::users)
• Problematic example showing conflicts with multiple DSL imports

This helps clarify one of the common pitfalls mentioned in the meta issue.

[Mahi-korrapati]

Hi! 👋
I’ve added the example for #[derive(Insertable)] in the docs to clarify common pitfalls with multiple DSL imports.
All checks are running/skipped as expected for docs-only changes.
Please review when you have a moment. Thank you! 😊