Updating README.md & CONTRIBUTING.md (and removing DEVELOPER_INSTRUCTIONS.md)

[tertsdiepraam]

I figured these documents where due for an update. Here are the justifications for the changes:

• I added the logo to the README because why not.
• Testing instructions are not useful for users, so I moved it all to CONTRIBUTING.md. The graph of the GNU tests is still on the readme.
• Believe it or not, but the first paragraph of the README didn't even mention the word "coreutils" so I fixed that. Also I think we've grown beyond calling uutils just an "attempt".
• The list of utilities always felt a bit weird to me, because nobody was really keeping track of it and the definitions of "done" and "wip" are very vague. I think issues are just a better way to keep track of what needs to happen. We could add a more comprehensive status page to the documentation as a replacement in the future.
• CONTRIBUTING.md started with instructions for adding new utils, which we no longer need, so I removed that and instead link to the good-first-issues.
• There was some duplicate info between DEVELOPER_INSTRUCTIONS and CONTRIBUTING, so I merged them. All tools like clippy, rustfmt, etc. are now mentioned in one place in a section called "Tools".

[sylvestre]

Excellent, thanks :)
Just some minor warnings in the CI needs to be fixed

[github-actions]

GNU testsuite comparison:

GNU test failed: tests/tail-2/inotify-dir-recreate. tests/tail-2/inotify-dir-recreate is passing on 'main'. Maybe you have to rebase?