Add codesandbox repl

[mattphillips]

What

Adds a codesandbox REPL for majority of the matchers

Notes

This PR adds conventions around how the README is generated, instead of being the source of truth, the docs folder is the source of truth with a readme.template.md which can interpolate links to code examples.

Example of the REPL in action:

https://codesandbox.io/s/github/jest-community/jest-extended/tree/docs-sandbox/docs/sandbox?module=%2Fmatchers%2Fstring%2FtoIncludeRepeated.test.js

TODO

Do something about all contributors as it will be amending the wrong README i.e. not the source of truth in /docs

Housekeeping

• [x] Documentation is up to date

[rickhanlonii]

This is really cool

[SimenB]

@mattphillips you gonna complete this? 😀

[SimenB]

@mattphillips ping