Proper handling of branches

[beta-ziliani]

Currently, there are two types of branches: those corresponding to a Crystal release (of the form X.X) and master (the latter called nighlty in the selector). The default landing page is the latest release.

Names are tricky business, so let's table this with some examples:

Selector name	Branch name (GH)	Link name	Crystal release	Default page
nighly	master	/master	master?
1.4	release/1.4	/1.4	1.4	✅
1.3	release/1.3	/1.3	1.3

Problem: Typically, all fixes and additions land on master (GH), but since this is not the default, they aren't immediately visible until the next release; something that currently might take up to three months. Since most of the fixes are valid in the latest Crystal release (1.4 in the example), it would be nice to not have to wait until the next release to have them visible.

Proposed solution: /master should really behave like nightly (and be called like that), and be the branch only for fixes relevant to Crystal's master and not applicable to the latest release. Then, by default a PR should land on the default page. In order to avoid having to sync different branches, a nightly run should merge the master branch with the nightly one.

Concretely:

Selector name	Branch name (GH)	Link name	Crystal release	Default page
nighly	nightly	/nightly	master
latest	master	/latest	1.4	✅
1.3	release/1.3	/1.3	1.3

Related #568

[straight-shoota]

I would name the branch that represents the current Crystal development version next. IMO that's more obvious because it's the documentation branch for the next release.