Feature docs example notebooks

[brynpickering]

Fixes #531 and adds notebooks back in to docs

Summary of changes in this pull request:

• Convert existing example/tutorial notebooks into python stripts using jupytext
• Move intro to example models into the notebooks (maybe a good idea maybe not?)
• Add plugin to render notebooks in docs
• Add extra css to better render xarray html outputs (see https://github.com/danielfrg/mkdocs-jupyter/issues/164)

Reviewer checklist:

• [ ] Test(s) added to cover contribution
• [ ] Documentation updated
• [ ] Changelog updated
• [ ] Coverage maintained or improved

[codecov[bot]]

Codecov Report

All modified and coverable lines are covered by tests :white_check_mark:

Comparison is base (5d81832) 95.18% compared to head (20ce565) 95.19%.

Additional details and impacted files

```diff @@ Coverage Diff @@ ## main #534 +/- ## ======================================= Coverage 95.18% 95.19% ======================================= Files 24 24 Lines 3305 3306 +1 Branches 706 652 -54 ======================================= + Hits 3146 3147 +1 Misses 92 92 Partials 67 67 ``` | [Files](https://app.codecov.io/gh/calliope-project/calliope/pull/534?src=pr&el=tree&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=calliope-project) | Coverage Δ | | |---|---|---| | [src/calliope/core/io.py](https://app.codecov.io/gh/calliope-project/calliope/pull/534?src=pr&el=tree&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=calliope-project#diff-c3JjL2NhbGxpb3BlL2NvcmUvaW8ucHk=) | `94.44% <100.00%> (+0.06%)` | :arrow_up: | | [src/calliope/core/model.py](https://app.codecov.io/gh/calliope-project/calliope/pull/534?src=pr&el=tree&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=calliope-project#diff-c3JjL2NhbGxpb3BlL2NvcmUvbW9kZWwucHk=) | `94.70% <100.00%> (ø)` | |