[Platform] Add docs for message templates

[chr-hertel]

Q	A
Bug fix?	no
New feature?	no
Docs?	yes
Issues
License	MIT

Follows #1017 - cc @wachterjohannes makes sense?

[Copilot]

Pull request overview

This PR adds comprehensive documentation for message templates in the Symfony AI Platform component. The documentation explains how to use dynamic variable substitution in messages for creating reusable message patterns.

Key Changes:

• Added documentation for string templates with curly brace placeholders
• Documented expression templates for advanced dynamic evaluation
• Included setup instructions for manual configuration

---

💡 Add Copilot custom instructions for smarter, more guided reviews. Learn how to get started.

[OskarStark]

I think we should add an example how you can configure templates in ai bundle config

[wachterjohannes]

@OskarStark in the ai-bundle you can just use it. but documenting how that looks like would make sense!

@chr-hertel should we document how the listener has to be configured when no ai-bundle is used? or link to the exanple?

[OskarStark]

Yes let's be explicit and explain it in the docs

[OskarStark]

Why closing ?

[chr-hertel]

oh, removed the branch on my fork because i thought it's merged ... :D

[chr-hertel]

I think we should add an example how you can configure templates in ai bundle config

@OskarStark currently there's nothing additional provided in the bundle - i added a section tho that the feature is just available.

should we document how the listener has to be configured when no ai-bundle is used?

@wachterjohannes there's a "Setup" section in the platform part