Add two-level UI and backend documentation

[yaniv-kwb]

Motivation

• Provide a concise, two-level overview of the project (UI vs Backend) to help contributors and users locate key entry points and understand the request lifecycle.
• Make the new overview discoverable from the docs homepage by linking it into the Sphinx toctree.

Description

• Add docs/source/application_layers.rst with two main sections: UI Layer documenting templates, static assets, JS snippets, and typical UI flow, and Backend Layer documenting Django settings, URLs, app structure, domain modules (models, forms, risk.py, plots.py, exports.py), and data bootstrap commands.
• Update docs/source/index.rst to include application_layers in the toctree so the new page appears in the documentation navigation.

Testing

• Ran a docs build with python -m sphinx -b html docs/source docs/build, which failed because the sphinx module is not installed in the current environment (automated test failed).

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Codex Task