New getting started doc

[arthurevans]

New draft getting started doc. Staged at:

https://new-getting-started-dot-polymer-lit-element.appspot.com

Leaving this PR open for feedback until Friday, 4/24.

[dman777]

Although I do not see it in this PR, I will mention it here since it is in the context:

In the example for events in docs:

class MyElement extends LitElement {
  render() {
    return html`<div>Hello World</div>`;
  }
  updated(changedProperties) {
    let click = new Event('click');
    this.dispatchEvent(click);
  }
}

Specifically, in let click = new Event('click').... this is incorrect JS. If click is not going to get reassigned so it should be using const.

Proof:

> const foo = { pie: 'apple' }
undefined
> foo.pie = 'blueberry';
'blueberry'
> foo = 'not going to happen';
Thrown:
TypeError: Assignment to constant variable.

[abdonrd]

@arthurevans as the PR is a draft, you can convert it here instead of adding draft to the title 😄

[arthurevans]

I swear I looked all over for that UI before I just gave up and edited the title, @abdonrd 😂

[arthurevans]

@dman777 you're right, but it seems unrelated... Do you want to open a PR?

[arthurevans]

Addressed feedback and restaged. Unless someone sees a showstopper, I'll roll this out Monday.

[arthurevans]

Thanks for all the great feedback, folks! Update is live.