Remove outdated MicroRust book

[pravic]

Resource description

This book https://droogmic.github.io/microrust/ was adopted and updated by https://docs.rust-embedded.org/discovery/microbit/, so no point in recommending the outdated book first - it is very confusing and people come and ask whether that resource is still useful or will be updated.

So, the newest book should be recommended instead.

Ideally, the new book should be placed at the top of the Rust category - since it is fundamental. And recommending various articles first is counter-productive.

By submitting this pull request I confirm I've read and complied with the below requirements 🖖

• I have read and understood the Contribution Guidelines
• I am making an individual pull request for each suggestion
• I have used the correct spelling and capitalisation for BBC micro:bit, micro:bit, and Micro:bit Educational Foundation (resource title could be excluded if there is a good reason)
• The content linked is in English, or it contains an English translation
• I have added the new entry to the bottom of its the category N/A
• I have used the following format:

- [Resource Title](link) - Resource short Description (2 lines or less in total)

[carlosperate]

It looks like https://droogmic.github.io/microrust/ contains a few things that are not quite covered in the discovery book, but I do agree that if they book is outdated and most of the content is covered there it's better to remove it.

The links are generally added chronologically, but it makes sense to move some resources to the top (or close to the top) in some cases, so I'll do that here.

Thanks @pravic!