Reorganize tutorials landing page (#3522)
Reorganize tutorials landing page (#3522)

As per discussion with @balopat and @lamberta, moves all GitHub examples to bottom of page, emphasizing on-site tutorials.

Additionally reorganizes some tutorials to balance out the sections. Before, there was 1 beginner, 1 intermediate, and 5 advanced tutorials. Now, there is 1 beginner, 3 intermediate, and 3 advanced tutorials organized as:

- Beginner
  - Basics
- Intermediate
  - VQE
  - QAOA
  - Quantum walks
- Advanced
  - Hidden linear function
  - Rabi oscillation
  - Shor's algorithm

I think this makes more sense content-wise in addition to balancing out the sections, but let me know what you think.
quantumlib/Cirq,balopat/Cirq,balopat/Cirq,quantumlib/Cirq,balopat/Cirq,quantumlib/Cirq,quantumlib/Cirq,quantumlib/Cirq