Implement unified docs for Lightning Qubit and Kokkos

[AmintorDusko]

Context:
Adds single unified documentation for Lightning Qubit and Lightning Kokkos.
For reference:
https://docs.pennylane.ai/projects/lightning/en/latest/
https://docs.pennylane.ai/projects/lightning-kokkos/en/latest/

Description of the Change:

Benefits:

Possible Drawbacks:

Related GitHub Issues:

[github-actions]

Hello. You may have forgotten to update the changelog!
Please edit .github/CHANGELOG.md with:

• A one-to-two sentence description of the change. You may include a small working example for new features.
• A link back to this PR.
• Your name (or GitHub username) in the contributors section.

[AmintorDusko]

This work is still under development. I'm tagging you just for your information.

[mlxd]

Just a quick request, the setup.py file needs a brief update for PyPI rendering, with the following change:

    "long_description_content_type": "text/x-rst",

[trbromley]

Other things we discussed:

• Here - we'll document the two modules, and then inside each module we'll have the LightningQubit etc content. Similar to how it's done here.
• Have sub-pages for installation
  • The lightning qubit one will include what's currently here
  • The lightning kokkos one can be more basic initially, but we can expand later on and also have room for lightning gpu
• Here and in the Kokkos equivalent, we'll add a sentence linking the user to the appropriate installation instructions.

[AmintorDusko]

Docs are ready, I think I covered all the topics we (Tom and I) discussed. I needed to adapt here the request below because our packaging structure is different from PennyLane and such a scheme would not work.

• Here - we'll document the two modules, and then inside each module we'll have the LightningQubit etc content. Similar to how it's done here.

I would like to ask for a final round of review.

[vincentmr]

I'll approve now, since I only have some questions and suggestions. This is a great (and important) job @AmintorDusko !

[trbromley]

Thanks @AmintorDusko!

[mlxd]

Let's go!