reorganize docs; add page for customizing gateway output

[ewhauser]

The existing site was a bit hard to navigate with all the pages at the top level. This change provides a better table of content for the docs and adds documentation for google.api.field_behavior.

I'm open to changing any terminology and structure. I don't love the current name for the Operations section as it's really "runtime behaviors + observability stuff".

[johanbrandhorst]

Amazing, can't thank you enough for this!

[bconway]

Thank you for all your work here and in #1806, this is great stuff.

I was a little surprised to find that field options did not show up when generating OpenAPI output, when following the instructions on the web site (Tutorial followed by Customizing OpenAPI Output). I then realized that this is not included as part of a release yet (go get/install downloads 2.0.1). Should a note be added to the doc regarding availability, or perhaps a new point release is expected in the near future?

Thanks.

[johanbrandhorst]

I've just tagged v2.1.0, thanks for the impetus.

[bconway]

Much appreciated!