docs: add Architecture & Developer Guide page with Mermaid flowchart

[Seai5]

Describe your changes

This PR adds a new Architecture & Developer Guide page (docs/developer-guide/architecture.md) with a clean, visual Mermaid flowchart explaining the full Neural-LAM pipeline.

The page includes:

• High-level overview with interactive-style Mermaid diagram
• Detailed explanation of each major step (Data Loading → Graph Construction → Model → Forecasting → Training)
• Tensor shape evolution notes
• Extension points for new features (new models, global forecasting, probabilistic ensemble support)

It will helps new contributors understand the codebase quickly.

Issue Link

#61

Type of change

☐ 🐛 Bug fix (non-breaking change that fixes an issue)
☐ ✨ New feature (non-breaking change that adds functionality)
☐ 💥 Breaking change (fix or feature that would cause existing functionality to not work as expected)
☑ 📖 Documentation (Addition or improvements to documentation)

Checklist before requesting a review

☑ My branch is up-to-date with the target branch.
☑ I have performed a self-review of my code
☑ For any new/modified functions/classes I have added docstrings that clearly describe its purpose, expected inputs and returned values
☑ I have placed in-line comments to clarify the intent of any hard-to-understand passages of my code
☐ I have updated the README to cover introduced code changes
☐ I have added tests that prove my fix is effective or that my feature works
☑ I have given the PR a name that clearly describes the change, written in imperative form (context).

This page can be easily rendered in Sphinx, MkDocs, or Jupyter Book.

Checklist for reviewers

• the code is readable
• the code is well tested
• the code is documented (including return types and parameters)
• the code is easy to maintain

Author checklist after completed review

☐ I have added a line to the CHANGELOG describing this change, in a section reflecting type of change (add section where missing):

• ☐ added
• ☐ changed
• ☐ fixes
• ☐ maintenance

[sadamov]

md file is broken and also the content is actually already covered by #202