Documentation for Got 12

[szmarczak]

https://github.com/sindresorhus/got/tree/docs-v12#documentation

Fixes #1405

TODO (the below will be another PR):

• update TypeScript docs
• Node.js migration guide
• Axios migration guide

Checklist

• I have read the documentation.
• I have included a pull request description of my changes.
• I have included some tests.
• If it's a new feature, I have included documentation updates in both the README and the types.

[sindresorhus]

I think we should preserve the initial usage example in the readme, so users can quickly see how easy Got is: https://github.com/sindresorhus/got#usage (JSON and Promise examples)

[sindresorhus]

@szmarczak Are you done with the docs? If so, I think it's easier to just merge it in and then I can do a look over and fix typos and stuff. It's faster for me to just fix things directly than to comment on things here.

[sindresorhus]

TODO (the below will be another PR):

Can you open an issue so we don't forget?

[szmarczak]

Are you done with the docs?

Yes.

Can you open an issue so we don't forget?

Sure.

[szmarczak]

#1684 (comment)