Bug: Duplicate entries for qiskit textbook / qiskit.org/learn

[HuangJunye]

Steps to reproduce the problem

Go to https://qiskit.org/ and click on "Documentation" to open the mega drop down menu

What is the current behavior?

There is already "Learn" in the top level item of the top menu. We don't need to have another entry of "Qiskit Textbook" in the drop down menu under "Help and Learning". I am not sure whether this was a conscious decision in #140

What is the expected behavior?

1. Remove "Qiskit Textbook" from "Help and Learning" column.
2. Rename "Help and Learning" column to "Get Help".

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Additional information

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