Documentation edits [JOSS REVIEW]

[PennyHow]

These are all suggestions to improve the package documentation, as part of my JOSS review.

• Introductory description of polartoolkit added to repo readme
• Introductory description of polartoolkit added to readthedocs
• Installation instructions added to repo readme

Feel free to merge and/or use this as a starting point to edit however you like. I'll write a more detailed justification of these changes in the JOSS review issue forum.

[welcome]

💖 Thanks for opening this pull request! 💖
Please make sure you read our contributing guidelines.
A few things to keep in mind:

• If you need help writing tests, take a look at the existing ones for inspiration. If you don't know where to start, let us know and we'll walk you through it. * All new features should be documented. It helps to write the docstrings for your functions/classes before writing the code. This will help you think about your code design and results in better code. * No matter what, we are really grateful that you put in the effort to do this! 🎉

[mdtanker]

@PennyHow I'm not exactly sure what happened here sorry! One of my Github actions was not working (I think since it was a PR from a fork which I hadn't tested yet), so I made about 10 different commits here to try to fix it. I gave up, and tried to drop those commits, but it seems I may have dropped all the commits here, closing this PR.

Are you able to open this again with your original commit please? Or just create a new PR with your same original changes?

[PennyHow]

Yeah sure, I'll open a new PR with the same changes.