Simplify language in the Release Channel section of Get Dart

[atsansone]

Page URL

https://dart.dev/get-dart#release-channels

Page source

src/get-dart/index.md

Describe the problem

The information within the About release channels and version strings section could use better organization. Recommend revising the content to better chunk the information into related blocks.

Expected fix

Recommend using the following copy.

### Stable channel
Dart deploys a release to the _stable_ channel about every three months.
The current stable version is  `[calculating]`{:.build-rev-stable}.

Use stable channel releases for production environments.

Versioning for releases in stable channel of the Dart SDK follows a `x.y.z` format.
The version strings resemble `1.24.3` and `2.1.0`.
This format consists of three dot-separated integers without hyphens or letters.
The versions include the major version (`x`), 
then the minor version (`y`), and then the patch version (`z`).

### Beta channel
Dart releases a build to the _preview_ channel about every month.
The current beta version is  `[calculating]`{:.build-rev-beta}.

These beta channel releases preview features intended for the stable channel.
Use releases in the beta channel to test new features or
test compatibility with future releases.
Don’t use beta releases for production environments. 

Versioning for releases in the **beta** channel of the Dart SDK
follows the `x.y.z-a.b.beta` format.
Version strings resemble `2.8.0-20.11.beta`.
The part before the hyphen follows the stable version format.
The part after the hyphen adds the pre-release version (`a`),
then the pre-release patch version (`b`), and then the channel (`beta`).

### Dev
Dart releases a build to the **dev** channel about twice a week.
The current dev version is  `[calculating]`{:.build-rev-dev}.

These dev channel builds include the most recent codebase with latest changes.
These builds may be broken, have no support,
and may contain unvetted breaking changes.

Versioning for releases in the **dev** channel of the Dart SDK
follows the `x.y.z-a.b.dev` format.
Version strings resemble `2.8.0-20.11.dev`.
Dev releases follow the beta channel versioning format replacing `beta` with `dev`.

Additional context

Originally raised as part of #4410.