React-select: Improve documentation

[gwyneplaine]

This is an Issue for us to keep track of ways we need to improve the documentation, feel free to add points in the comments below.

• Add example for getOptionValue method
• Add documentation around the expected default properties of options (value: [any], label: string).
• Add Async Select section to the upgrade-guide
• #3037

[danpoq]

I'm not sure whether this requires its own issue but right now there could be a potential problem with disparity between the contents of the docs site and the npm release.

Got quite confused just now because https://react-select.com/styles#overriding-the-theme is live. I had to go through issue history to find that the PR is merged but there's no npm release.

Seeing as though docs are hosted from docs/ on master. One solution might be to add something like Since 2.0.1 to each new feature in the docs. Might also be a larger problem with your branch management e.g. master = latest released stable version.

[wejrox]

The source links are pointing to the wrong URL for the examples. Presumably they have changed and not been updated.

[Rall3n]

@wejrox https://jedwatson.github.io/react-select/ has been replaced with https://v1.react-select.com. There all example links are pointing to the correct source.