Add ADM1 Implementation and Substrate Modeling Documentation

[dgaida]

I have added a comprehensive documentation page describing the ADM1 implementation and substrate modeling in this repository.

Summary of Changes

1. New Documentation Page: Created docs/de/user_guide/adm1_implementation.md and its English counterpart docs/en/user_guide/adm1_implementation.md. These files:
   • Explain how the model differs from the standard IWA ADM1 (pure ODE, kinetic acid-base/gas-transfer handling).
   • Detail the 37 state variables tracked.
   • Describe the methodology for land-agricultural substrate modeling via Weender and Van Soest fractions.
   • Explain the dynamic calculation of composite material composition ($X_c$) and kinetic rates based on the feed mix.
2. MkDocs Navigation: Updated mkdocs.yml to include the new page under the "Dokumentation" section for both German and English users.

Verification

• Content Review: The content was written based on the provided technical references (PhD thesis Gaida 2014, standard ADM1 documentation, and C# source code logic).
• Documentation Build: Ran mkdocs build to ensure the new files were correctly picked up and that the navigation was correctly structured without introducing build errors.
• Code Review: The documentation was reviewed and confirmed to be correct and high-quality.

---

PR created automatically by Jules for task 8042139006703194449 started by @dgaida

[google-labs-jules]

👋 Jules, reporting for duty! I'm here to lend a hand with this pull request.

When you start a review, I'll add a 👀 emoji to each comment to let you know I've read it. I'll focus on feedback directed at me and will do my best to stay out of conversations between you and other bots or reviewers to keep the noise down.

I'll push a commit with your requested changes shortly after. Please note there might be a delay between these steps, but rest assured I'm on the job!

For more direct control, you can switch me to Reactive Mode. When this mode is on, I will only act on comments where you specifically mention me with @jules. You can find this option in the Pull Request section of your global Jules UI settings. You can always switch back!

New to Jules? Learn more at jules.google/docs.

---

For security, I will only act on instructions from the user who triggered this task.