docs: update README with intro_to_FEA

[kubaniak]

Update the README to include the intro_to_FEA project. The entry was added in alphabetical order with a description formatted to match the existing entries.

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PR created automatically by Jules for task 2693498161553376255 started by @kubaniak

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[Copilot]

Pull request overview

Updates the repository README to include the new intro_to_FEA mini-project in the directory summary.

Changes:

• Added a new README entry for intro_to_FEA, placed in alphabetical order.
• Matched the existing description formatting and style used for other directories.

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