Document top-level code layout in README

[Copilot]

The issue was a basic code-readability gap: the repository did not provide a quick map of where the main implementation lives. This change adds a concise top-level layout summary so readers can find core library code immediately.

• README navigation

  • Added a Code layout section near the top of README.md
  • Calls out the main package areas and their roles:
    • femo/fea/ for finite-element wrappers and derivative utilities
    • femo/csdl_opt/ for CSDL integration models
    • examples/ for end-to-end usage
    • docs/ for Sphinx sources

• Result

  • New contributors can orient themselves in the codebase without opening multiple directories first
  • Improves discoverability without changing package behavior

## Code layout

- `femo/fea/`: core finite-element wrappers, PDE solves, and derivative utilities built around FEniCSx.
- `femo/csdl_opt/`: CSDL-facing models that expose states, outputs, and optimization couplings.
- `examples/`: end-to-end problem setups showing how to assemble and run the framework.
- `docs/`: Sphinx documentation sources.

Original prompt

can you read my code

Created from VS Code.

[Copilot]

Pull request overview

This PR improves repository discoverability by adding a concise “Code layout” map near the top of README.md, helping new contributors quickly locate the main implementation areas (core finite-element logic, CSDL integration, examples, and documentation).

Changes:

• Added a ## Code layout section to the README.
• Documented the purpose of femo/fea/, femo/csdl_opt/, examples/, and docs/ at a high level.

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