Add docs for building Python bindings with built-from-source library

[hello-binit]

This is a small PR to improve the docs for building the Python building for MuJoCo. The existing docs explain how to use a MuJoCo binary from GitHub while building the bindings. With this addition, it also explains how to build the library from source and use it with the Python bindings.

[google-cla[bot]]

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[yuvaltassa]

Can you do the CLA thing please?

[hello-binit]

Done!

[nimrod-gileadi]

Looks like the CLA check is still failing.