add rustdoc and clippy to github workflow

[hellow554]

This commit is quite a mouth full and I'm can understand if you want me to split this into smaller chunks.

Let me start explain it commit by commit.

First I created a clippy toml with the msrv set to 1.36.0, which is the version used in the github workflow.

Next I either fixed all the linted lines, or added a allow with a comment to it.
I did the same for all the tests.

Next I saw, that there is currently a rustfmt which skips the complete source base, based on a commit two years ago: 641671e

"Temporarily disable" 😉

I then reformatted the whole codebase, which might break some PRs but they should be able to rebase it but just using cargo fmt easily.

Next, I fixed two warnings. One stray ``` and the other one being changed in rust-lang/rust#96676 where you have to specify a macro link.

Last, but not least, I added rustfmt rustdoc and clippy to the ci.yml to enable them by default on PRs.

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Feel free to comment whatever you think. I try to answer or change it :)

Thanks for the awesome crate :)