Update build instructions following move to Poetry

[FroggyFlox]

Fixes #405 @phillxnet, ready for review.

This pull request's proposal

Update our contribution instructions following our move to Poetry (04a7d21524e3a32ca5ab5c4d387bd9bbd8a8a43e). This PR also includes minor typos and style adjustments (a069da9333387e6cb6291a4189364ee2deddb821) in an attempt at making the text more fluent and thus hopefully a bit easier to follow. I hope it actually achieves that.

@phillxnet, I've tried to separate the changes into two commits to ease review; commit 04a7d21524e3a32ca5ab5c4d387bd9bbd8a8a43e should include all changes related to the move to Poetry.

Checklist

• [x] With the proposed changes no Sphinx errors or warnings are generated.
• [x] I have added my name to the AUTHORS file, if required (descending alphabetical order).

[FroggyFlox]

Thanks @phillxnet for your thorough review; I included all your suggestions.

[phillxnet]

PR product PRODUCTION published.