Adds documentation site

[BradyPlanden]

This PR adds a documentation site using sphinx and the pydata-sphinx-theme. To accomplish this, I've updated the docstrings for the package where possible (let me know if I've missed any). As previously discussed, I've relocated the costs and dataset files to the first level of the package.

• Update docstrings across the package for correct rendering in sphinx (should align with PEP-0257)
• Add a deployment workflow: deploy-docs.yaml
• Add a nox session for live-editting of the docs: nox -s docs
• Moves cost and dataset classes to pybop/ as _costs.py and _dataset.py
• Adds a pip installation flag for docs: pip install -e .[docs]
• Update changelog.md and contributing.md

Closes #15. Live site can be viewed at: https://pybop-team.github.io/PyBOP/

[codecov]

Codecov Report

Attention: 5 lines in your changes are missing coverage. Please review.

Comparison is base (707f16a) 93.03% compared to head (af10b31) 93.03%.
Report is 8 commits behind head on develop.

❗ Current head af10b31 differs from pull request most recent head a0d44e5. Consider uploading reports for the commit a0d44e5 to get more accurate results

Files	Patch %	Lines
pybop/_dataset.py	63.63%	4 Missing ⚠️
pybop/optimisers/base_optimiser.py	0.00%	1 Missing ⚠️

Additional details and impacted files

@@           Coverage Diff            @@
##           develop     #141   +/-   ##
========================================
  Coverage    93.03%   93.03%           
========================================
  Files           34       34           
  Lines         1178     1178           
========================================
  Hits          1096     1096           
  Misses          82       82           

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[NicolaCourtier]

Really helpful PR! The documentation looks great and the comments are clear and informative. I've added a few minor comments, but overall excellent work bringing the documentation up to scratch, thanks @BradyPlanden!