Change docs to use Pydata theme

[valentinsulzer]

Description

This is the theme used by numpy, scipy, pandas, etc and looks much better than our current one.
Also did a little bit of reformatting, started a user guide, etc, but there is more to be done

Type of change

Please add a line in the relevant section of CHANGELOG.md to document the change (include PR #) - note reverse order of PR #s. If necessary, also add to the list of breaking changes.

• New feature (non-breaking change which adds functionality)
• Optimization (back-end change that speeds up the code)
• Bug fix (non-breaking change which fixes an issue)

Key checklist:

• No style issues: $ flake8
• All tests pass: $ python run-tests.py --unit
• The documentation builds: $ cd docs and then $ make clean; make html

You can run all three at once, using $ python run-tests.py --quick.

Further checks:

• Code is commented, particularly in hard-to-understand areas
• Tests added that prove fix is effective or that feature works

[codecov]

Codecov Report

Base: 99.69% // Head: 99.69% // Increases project coverage by +0.00% 🎉

Coverage data is based on head (58afd08) compared to base (3359e13).
Patch has no changes to coverable lines.

❗ Current head 58afd08 differs from pull request most recent head 00e2df8. Consider uploading reports for the commit 00e2df8 to get more accurate results

Additional details and impacted files

@@           Coverage Diff            @@
##           develop    #2618   +/-   ##
========================================
  Coverage    99.69%   99.69%           
========================================
  Files          271      271           
  Lines        19570    19571    +1     
========================================
+ Hits         19510    19511    +1     
  Misses          60       60           

Impacted Files	Coverage Δ
pybamm/models/submodels/electrode/ohm/li_metal.py	100.00% <ø> (ø)
pybamm/__init__.py	100.00% <0.00%> (ø)
pybamm/input/parameters/lithium_ion/Ai2020.py	100.00% <0.00%> (ø)
...i2020/graphite_entropy_Enertech_Ai2020_function.py	100.00% <0.00%> (ø)

Help us with your feedback. Take ten seconds to tell us how you rate us. Have a feature suggestion? Share it here.

☔ View full report at Codecov.
📢 Do you have feedback about the report comment? Let us know in this issue.

[valentinsulzer]

Docs look like this on latest build: https://pybamm--2618.org.readthedocs.build/en/2618/
Don't know why the pydata theme isn't being used

[valentinsulzer]

Almost ready to go, final problem is the code is not displaying on this page https://pybamm--2618.org.readthedocs.build/en/2618/source/user_guide/fundamentals/index.html (it's not displaying locally either)

[valentinsulzer]

Found the problem, I was doing .. code-block::python instead of .. code-block:: python 🤦

[Saransh-cpp]

The documentation looks amazing! Thanks, @tinosulzer!

[Saransh-cpp]

Was this intentional?

[valentinsulzer]

yes, the contents of that file are now in quick_plot since they go together

[brosaplanella]

Looks great, thanks! We should aim to merge this by Tuesday so we can have the new docs with the new release.