[BUG] Rendering issues

[josh146]

The following demos include incorrect ReST, that may result in rendering issues.

Title underline too short

• tutorial_adaptive_circuits.rst

  Sparse Hamiltonians
  -------------
  

• tutorial_sc_qubits.rst

  Quantum computing with superconducting qubits
  =============================
  

• tutorial_tn_circuits.rst

  PennyLane Implementation
  ^^^^^^^^^^^^^^^^
  

• tutorial_tn_circuits.rst

  PennyLane Implementation
  ^^^^^^^^^^^^^^^^
  

• tutorial_vqe_spin_sectors.rst

  Building the Hamiltonian and the total spin operator :math:`\hat{S}^2`
  ----------------------------------------------------------------
  

Invalid meta directive

This one could be causing the classical kernels demo to have unlisted metadata.

• tutorial_classical_kernels.py

  .. meta::
      :property="og:description": Finding a QK to approximate the Gaussian
      kernel.
      :property="og:image": https://pennylane.ai/qml/_images/toy_qek.png
  

Figure and image issues

• tutorial_haar_measure.py: WARNING: Figure caption must be a paragraph or empty comment.

  .. figure:: /demonstrations/haar_measure/spherical_int_dtheta.png
      :align: center
      :width: 50%
  

Incorrect or broken links

• tutorial_pasqal.rst WARNING: Duplicate explicit target name: "cirq".

• tutorial_pasqal.rst: WARNING: Duplicate explicit target name: "pennylane-cirq plugin".

• demos_qml.rst WARNING: toctree contains reference to nonexisting document 'demos/tutorial_ensemble_multi_qpu'

• demos_quantum-chemistry.rst WARNING: toctree contains reference to nonexisting document 'demos/tutorial_vqe_parallel'

• demos_quantum-computing.rst WARNING: toctree contains reference to nonexisting document ' demos/tutorial_sc_qubits'

• demos_submission.rst WARNING: document isn't included in any toctree

• glossary/automatic_differentiation.rst WARNING: document isn't included in any toctree

General rendering issues

• demos_quantum-computing.rst:80: WARNING: Explicit markup ends without a blank line; unexpected unindent.

• tutorial_mol_geo_opt.py WARNING: Line block ends without a blank line.

[ikurecic]

I'm in the process of fixing some of these in the context of cleaning up some formatting, referencing, and authorships: #607

[ankit27kh]

Issue with this image in givens rotation tutorial

Also, the references are not hyperlinks

[ikurecic]

Thanks a lot for the ping, @ankit27kh ! ☺ We'll update this demo right away.