[docs] Add a gif of optimal DOM updates

[justinfagnani]

This could be on the home page, introduction, concepts, or all of the above.

Similar to: https://reactjs.org/docs/rendering-elements.html#react-only-updates-whats-necessary

[brettdewoody]

I could see this living in at least 2 places:

• the end of the Getting Started, after the Rendering section
• in the Render Dynamic Text Content section of Writing Templates
• others?

We can create a lit-html version of the React date rendering example for an apple-to-apples comparison, perhaps with a section title of 'lit-html Only Updates What's Necessary' and a quick explanation of what's happening. Here's a Codepen with the same example using lit-html.

And here's a gif showing the updates (slightly sped up):

[brettdewoody]

@justinfagnani what are your thoughts on the above?