Duplicate instructions for "installation from source" in `getting_started.rst` and `contributing_to_qiskit.rst`

[jond01]

These referenced labels from the file contributing_to_qiskit.rst (L656-L660)

#. :ref:`qiskit-terra <install-qiskit-terra>`
#. :ref:`qiskit-aer <install-qiskit-aer>`
#. :ref:`qiskit-ignis <install-qiskit-ignis>`
#. :ref:`qiskit-aqua <install-qiskit-aqua>`
#. :ref:`qiskit-ibmq-provider <install-qiskit-ibmq-provider>`

are duplicated in getting_started.rst (L131-L135):

   #. :ref:`qiskit-terra <install-qiskit-terra>`
   #. :ref:`qiskit-aer <install-qiskit-aer>`
   #. :ref:`qiskit-ignis <install-qiskit-ignis>`
   #. :ref:`qiskit-aqua <install-qiskit-aqua>`
   #. :ref:`qiskit-ibmq-provider <install-qiskit-ibmq-provider>`

The "installation from source" instructions are duplicated in two places with the same labels, so when clicking link 1 here: https://qiskit.org/documentation/contributing_to_qiskit.html#installing-from-source One is redirected to: https://qiskit.org/documentation/getting_started.html#install-qiskit-terra Instead of: https://qiskit.org/documentation/contributing_to_qiskit.html#installing-terra-from-source

Originally posted by @jond01 in https://github.com/Qiskit/qiskit/issues/1351#issuecomment-963281590