docs/getting-started: Update pip environment build dependency

At least 2 packages are built during the initial pip install:

• subvertpy
• dulwich

Thus, we need the Python.h library.

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I also created an issue on subverty project to ask for wheels https://github.com/jelmer/subvertpy/issues/28

and there is https://github.com/dulwich/dulwich/issues/222 on the dulwich side