New CI

[marioortizmanero]

This PR includes the new CI suggested in #213. It's merged over #215 to avoid merge conflicts later on.

[marioortizmanero]

I've been taking a look at Bevy's CI and maybe we could copy some of their stuff? I'm taking a look right now.

[marioortizmanero]

OK so I've added a flow that checks for broken links in the docs as well. This PR fixes all of them except for those that are impossible to fix until everything's merged. For example I added links to the new examples like https://github.com/ramsayleung/rspotify/blob/master/examples/auth_code.rs, but they won't work until the rewrite hits the master branch (they work with https://github.com/ramsayleung/rspotify/blob/auth-rewrite-part3/examples/auth_code.rs for now)

As to having a flow for each crate I'm not sure if it's worth it. Specially because the macros crate doesn't have that many tests, for example. I think having a flow for each client might be enough for our case.

[marioortizmanero]

After a lot of debugging, turns out it's very complicated to have a clean run of cargo-deadlinks (i.e. without errors). For example, ours fails due to an external crate, tokio-rs/tracing#1078.

Thus, I'm adding a cargo-deadlinks flow, but it will continue on error for now. This is what Bevy does as well, so it makes sense to me. It's still useful to check from time to time and it avoids unfixable errors.

This is ready for review, @ramsayleung

[ramsayleung]

Do we just check dead links for cli and env-file features, with ignoring other features?

[marioortizmanero]

The rest of the features don't add anything new to the docs. Here I just generate the docs with everything possible in order to check for dead links.

[ramsayleung]

I have to say, the cargo-deadlinks is great, it helps us find the broken links, instead of identifying it manually.