Add a "was this page helpful" widget to docs pages

## Why are we implementing it? (sales eng)

### What are the typical use cases?

When people are browsing the docs we'd like to get their feedback while it's on their mind. Currently the feedback cycle is kind of indirect.

## Communication goals (e.g. detailed howto vs orientation)

### Good locations for content in docs structure

Customize the theme via `_templates/page.html` to include code for the widget. Investigate if there is a pre-made service for measuring this kind of feedback.