Update .readthedocs.yaml for pyproject.toml build system

[Copilot]

• Analyze current .readthedocs.yaml configuration
• Understand the new build process (pyproject.toml + custom build backend)
• Update .readthedocs.yaml to:
  • Remove references to setup.py (build_cython, build_ext)
  • Remove reference to non-existent requirements.d/rtd.txt
  • Remove unnecessary git fetch --unshallow job
  • Keep pip install with path which will use pyproject.toml
  • Ensure system packages remain for Cython extension build
• Run code review and security checks
• Address reviewer feedback

Original prompt

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This section details on the original issue you should resolve

<issue_title>Update readthedocs config</issue_title>
<issue_description>https://github.com/libfuse/pyfuse3/blob/main/.readthedocs.yaml needs an update to work with the updated build process (no more setup.py).</issue_description>

<agent_instructions>Update .readthedocs.yaml so that it works again, developer-notes/release_process.rst should be helpful.</agent_instructions>

Comments on the Issue (you are @copilot in this section)

• Fixes Update readthedocs config #119

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[Nikratio]

Otherwise LGTM.