Add Netlify integration for DOM Fixtures folder

[klamping]

In issue #11079, @nhunzaker mentions that it would be nice to have an automatic deployment of the DOM Fixtures folder for manual and automated testing purposes.

Since Netlify offers their services for free to open source projects (an account for this will need to be set up), I thought it would be a great fit.

This commit adds a netlify.toml file to the repo, allowing easier deployment out to the service.

This file will do two things:

1. On master branch update, it deploys the latest code to the main linked netlify site. For example: https://musing-lewin-a6efa4.netlify.com/
2. For every PR opened, creates a 'Deploy Preview' site for validating that PR's changes. For example: https://deploy-preview-1--musing-lewin-a6efa4.netlify.com/
   The preview site is linked via GitHub in the PR.
   

Test Plan:
Create a Netlify account and connect the github repo you're testing to it.
After connection, Netlify should deploy the DOM fixtures build folder to a live site for you to use.

[aweary]

This is very cool, thank you for doing this @klamping! The React website is powered by Netlify, so we may already have an official account. cc @gaearon

[nhunzaker]

Wow. This is so cool; thank you so much!!

Let's get the account stuff setup!

[jquense]

awesome!

[klamping]

Any chance I can get an update on the status of this?

[gaearon]

@klamping Hey sorry for no response, we're pretty swamped by some longer term projects and so everything related to infra like testing is on a backburner. We really appreciate the PR but realistically we don't have enough time to review DOM-related changes either so it's not urgent to get in. When we get closer to working on these changes, we'll definitely check it out. Thanks!

[klamping]

@gaearon thanks for getting back with me and the explanation on it all!