misc: use tfplugindocs to generate docs.

[jmcarp]

Rather than constructing docs manually and running them through a linter, use tfplugindocs to generate them. We move examples into the examples/ directory, and override default templates in the templates/ directory. We also add new make targets to generate docs and check that the generated docs match the code.

Resolves #490.

Note: this is a low priority, but I wanted to fix the paper cut before investing more time in the provider.

Also note: the generated docs aren't identical to the current docs. I haven't gone through and tried to make them as similar as possible, but we can do more to minimize the diff if we want to.

[sudomateo]

I really like this idea! I haven't used tfplugindocs so I want to better understand how it expects to use templates and spend some time updating our descriptions first so the generated documentation is consistent and useful. Agree it's low priority but I'll try to spend some time looking at this on the plane when I travel next week.