[docs] Docs feedback mechanism

[arthurevans]

Another general design request for post-1.0: I'd like to provide a feedback mechanism. We can easily put one at the bottom of the page, but since some of our pages are quite long, having a "was this page helpful" button at the very end isn't perfect.

Old Polymer site had "Edit on GitHub" buttons, which were nice, but much higher friction, since users had to have a GitHub account, be willing to click through and create a PR, instead of just commenting anonymously.

Options include:

• A sticky footer (see the Sentry docs.
• Something attached to the header.
• A small feedback tab on the side that opens an overlay. I'm not saying this well, but I've seen that implemented somewhere, too.

Information to collect:
Was this page helpful to you? [Yes] {No]
[If no] Comment [ ]
Could also offer "Edit on GitHub" or "Open an Issue" as options.

[arthurevans]

cc: <removed by @e111077 >

[e111077]

Contacted ocupop on this

[e111077]

copying @justinfagnani's comment from duplicate:

This would also be good to have for bugs in the playground implementation, and feedback on the playground examples.

MDN does a good job of pre-populating issues from their "report an issue with this page" button. We could also enable a link that takes the user directly to editing the file in GitHub.

[e111077]

How's this for a first attempt?

#919