Deploy master docs to GitHub Pages

[daxpedda]

This deploys documentation generated from the master branch to GitHub Pages.
I also added the extra couple of lines needed to scrape examples, which should also be visible on docs.rs.

[madsmtm]

I fear this may confuse users on which docs they need to look at (which is almost always the published one on docs.rs), so unless you have some grander plan with this, I think I'd rather not do this?

[daxpedda]

I fear this may confuse users on which docs they need to look at (which is almost always the published one on docs.rs), so unless you have some grander plan with this, I think I'd rather not do this?

The only place this is exposed to anybody is a badge on the README saying "master docs" right next to the badge saying "docs", I could improve the "docs" badge saying "crates.io docs". I find this very clear.

But if this is still not clear enough, I can just bury it somewhere else in the README saying "here are the docs to the current master branch: [URL]".

The purpose here is to increase the visibility of the documentation we are currently working on and exactly how it looks like, especially after #3076.

[notgull]

LGTM to me. This is done in a couple of other repos whose names escape me at the moment. Since we have a few users using the master branch of winit this is probably helpful

[madsmtm]

The only place this is exposed to anybody is a badge on the README saying "master docs" right next to the badge saying "docs", I could improve the "docs" badge saying "crates.io docs". I find this very clear.

That's probably fine then, I didn't check that beforehand