Add documentation for autogen packages maintained by googleapis team

[bjwatson]

Updates #1384

[bjwatson]

FYI @geigerj and @omaray

[dhermes]

1. I'm surprised you want this index in here. I expected it'd be in the docs for GAX or somewhere else.
2. What sort of process will keep this doc from going stale?

[bjwatson]

@dhermes Good questions:

1. I could move it to https://github.com/googleapis/googleapis/. That also makes sense as a place to keep this. Your preference, since y'all are the primary customer for this doc.
2. Mostly this doc will be additive, except when we sunset an API (probably years from now). As the GAX Python TL, I'll take ownership of keeping this doc up to date, regardless of where it's located.

[bjwatson]

Correction to (2): There will be updates when we rev a major version of an API, which I will have to keep in mind when updating the GAX wrapper.

[dhermes]

I'm inclined to say put it in https://github.com/googleapis/googleapis/ but I'll let others weigh in

[tseaver]

That repo SGTM: it is where I would look for those docs if I weren't elbows-deep here every day.

[dhermes]

OK @bjwatson shall we close out?

[bjwatson]

@dhermes SGTM