feat: user-defined rules page

[deemp]

Closes #119

Render

1. Get mdbook. E.g., nix shell nixpkgs#mdbook
2. Run it in the site/docs directory

cd site/docs
mdbook serve

3. Open the address in a browser

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PR-Codex overview

This PR introduces user-defined rules for the PHI normalizer.

Detailed summary

• Added a new file user-defined-rules.md to the documentation.
• Added a section on user-defined rules in the user-defined-rules.md file.
• Added an image rules.jpg to the media folder.
• Added a sample program program.phi in the common folder.
• Added a section on normalizing PHI options in the normalize-phi-options.md file.
• Added command examples for using user-defined rules in the user-defined-rules.md file.

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