Add more examples to bokehjs

[mattpap]

Existing examples don't show much. We need to show a reasonable selection of features. This is related to issue #5877.

This should include:

• plotting API
• categorical axes (PR Fix bokehjs' plotting API and add categorical example #6647)
• layouts
• widgets

[David-Herman]

Thank you, Also perhaps you can include elements of my existing questions.
https://stackoverflow.com/questions/44784697/bokehjs-title-text-alignment

[mattpap]

A new set of examples was added under bokehjs/test/integration/examples across multiple recent PRs. Those examples are under visual tests, the same as we use for visual integration testing.