added mise-en-place

[jdx]

Because mise is the recommended install in the new rails guides it makes sense to at least include it here.

rvm and rbenv are explicitly called out here as well, not sure if mise is important enough to be added there or not:

www.ruby-lang.org/en/downloads/index.md

Lines 16 to 19 in 989940d

	* On Linux/UNIX, you can use the package management system of your
	distribution or third-party tools ([rbenv][rbenv] and [RVM][rvm]).
	* On macOS machines, you can use third-party tools ([rbenv][rbenv] and [RVM][rvm]).
	* On Windows machines, you can use [RubyInstaller][rubyinstaller].

I feel like with each of the tools mentioned it would be nice to break the operating system support out into something common like this, but I was curious what y'all think before I added this:

### mise-en-place
{: #mise-en-place}

**OS:** UNIX-like, Windows

[mise-en-place][mise-en-place] allows you to switch between multiple Rubies.

I'm happy to also add this to the translated pages once we're happy with the English text—though the only language I'll be able to do without blindly using Google translate will be French.

[jdx]

not sure if this is too wordy, I felt like if I mention ruby-build/ruby-install that may confuse users thinking they need to first install those which they don't (in fact mise can't even be configured to use an existing ruby-install/ruby-build)