News and compatibility annotations for Julia 1.1

[fredrikekre]

Me and @mortenpi have gone through the ~570 PRs that have been merged since Julia 1.0 (https://docs.google.com/spreadsheets/d/1PThPlhFwlkcmmEpe8JcJ-XyTdU-I1lmEh7iCw8AvFSM/edit?usp=sharing20) in preparation for the 1.1 release. We have added news and compatibility annotations for changes worthy of that.

Since we now have one doc for all of Julia 1.X (#26825) we have thus added small !!! compat notes to the manual for e.g. new functions and new behaviors. For a taste of what these annotations look like see e.g. https://julialang.github.io/Pkg.jl/dev/registries/

Walking through the PRs like this is not very efficient (and pretrty boring), so for future features I ask people to add these annotations together with NEWS.

[StefanKarpinski]

OMG, you two are saints!

[mortenpi]

so for future features I ask people to add these annotations together with NEWS.

Should we add a note to CONTRIBUTING.md? Something along the lines of:

#### Annotations for new features, deprecations and behaviour changes

API additions and deprecations, and minor behavior changes are allowed in minor version releases. For documented features that are part of the public API, a compatibility note should be added into the manual or the docstring. It should state the Julia minor version where the change was made and have a brief message describing the change.

At the moment, this should always be done with the following `compat` admonition (so that it would be possible to programmatically find the annotations in the future):

```
!!! compat "Julia 1.X"
    This method was added in Julia 1.X.
```

[JeffBezanson]

This is amazing. I was just starting to worry about how we were going to do this for this release and who was going to do it (me?) but now here it is :)

[StefanKarpinski]

I cannot emphasize how wonderful it is that you two did this. Thank you sooo much! ❤️❤️❤️

[mortenpi]

@mbauman suggested using a different icon (arrow-circle-up) for the compat notes. For that we just need to add this to the CSS:

.admonition.compat > .admonition-title:before {
    font-family: "FontAwesome";
    content: "\f0aa";
}

The same change needs to be done in Pkg as well to keep the style consistent.

[mbauman]

It's not a very strong preference — which is why I figured I'd raise the possibility on slack instead of here. My thought is just to differentiate things a bit more from warnings, but the green does a pretty good job of that. And we also use the encircled ! for !!! notes, too, so let's not worry about the color of this 🚲🏠 too much.