[WIP] docs(generator): use NextJS to generate prerendered pages

[layershifter]

WIP

Fixes #2826.

[layershifter]

@levithomason can you take a look and give some feedback?

This is an early POC, you will need to run:

yarn next:build
yarn next:serve

I don't integrated it deeply to our build process, yet. There are only two working pages:

• theming: http://localhost:3000/theming
• usage: http://localhost:3000/usage

The generated pages will be also in the out directory, so you will can review them.

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If we will use NextJs we should make changes in the directory structure:

docs/src/views => pages
docs/src/assets => static

[layershifter]

I tried react-static and seems that it feets better, working on new PR.