rename classic_docs to something more indicative of what they are making code maintainance of doc building easier

Update sphinx packages used to build docs make petsc4py docs shared common requirements.txt

This prevents errors such as below when building docs

Running Sphinx v7.3.7
Using PETSC inventory from fi

Update sphinx packages used to build docs make petsc4py docs shared common requirements.txt

This prevents errors such as below when building docs

Running Sphinx v7.3.7
Using PETSC inventory from file:///Users/barrysmith/Src/petsc/doc/_build/html/objects.inv
<Inventory (url): PETSc v3.21, 7842 objects>

Exception occurred:
File "petsc4py/PETSc.pyx", line 1, in init petsc4py.PETSc
ValueError: numpy.dtype size changed, may indicate binary incompatibility. Expected 96 from C header, got 88 from PyObject

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