Dynamic Routes and Data Fetching

## 🚀 Feature

Support "nextjs-like" dynamic routes so that N pages may be generated from pages with templated filenames (`[thingid].tsx`).

### Have you read the [Contributing Guidelines on issues](https://github.com/facebook/docusaurus/blob/master/CONTRIBUTING.md#reporting-new-issues)?

yes

## Motivation

Project I'm working on has a bunch of static json data from which we'd like to seed multiple pages. People on the project are super torn because Docusaurus offers so much ease and flexibility for everything we need, but skeptical of doing pre-build generation ourselves via a plugin, etc.

## Pitch

Support Dynamic Routes and Data Fetching ala NextJS:

- https://nextjs.org/docs/basic-features/data-fetching
- https://nextjs.org/docs/routing/dynamic-routes

In NextJS, you just make a file with a templated filename, slap a `getStaticProps` and `getStaticPaths` functions in the module and you're good to go. If Docusaurus had this feature, when you would ever need to actually reach for NextJS?! :)