Move usage guides to their own docs.

[tswast]

The API reference links were buried behind pages of
code samples. Moving the how-to guides to their own
pages makes the documentation easier to navigate.

[tseaver]

I have a question, rather than a direct review comment: would it be better to call the section usage than howto? Right now we have a page, /usage.html, which redirects to /index.html: that suggests that usage is part of the expected documentation pattern.

[tswast]

Would it be better to call the section usage than howto?

I chose howto because that's in line with our GCP tech writing style guide, but you're right that usage.html is the existing pattern in this library. Maybe I should move the subsections that I put in index.rst to their own usage/index.rst file?