qml [BUG] Rendering issues

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.