Improve Developer Experience with Zero-Config Auth and Diátaxis Docs

[dcode]

This PR significantly improves the Developer Experience (DX) of the prusa-connect library.

Key Changes:

1. Zero-Config Authentication:

   • PrusaConnectClient() can now be instantiated without arguments. It will automatically look for credentials in:
     1. Environment variables (PRUSA_TOKENS_JSON or PRUSA_TOKEN).
     2. A default prusa_tokens.json file in the current directory (which is where the CLI saves them).
   • If no credentials are found, it raises a descriptive PrusaAuthError guiding the user to the CLI.

2. Documentation Overhaul:

   • Rewrote README.md following the Diátaxis framework.
   • Added a "Quickstart" tutorial that leads to a "Hello World" in minutes.
   • Added "How-to Guides" for headless auth, controlling printers, and more.
   • Included copy-pasteable snippets for common tasks.

3. DX Improvements:

   • Added from_env factory to PrusaConnectCredentials to support CI/CD usage more easily.
   • Updated from_file to accept str paths as well as Path objects.

Verification:

• Added tests/test_dx.py to verify the auto-loading logic and environment variable support.
• Ran existing tests to ensure no regressions.
• Verified that lint checks pass.

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PR created automatically by Jules for task 15224990671937316957 started by @dcode

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