robots.txt for Gemini formalised

[Natalie Pendragon]

This looks great! I'm excited to see this companion spec become more
formalized, and really like the categorical virtual agent design. One
thing that stuck out to me after a first read was the `webproxy` user
agent. What would you think of something like the following instead:

`proxy`
`proxy-web`
`proxy-gopher`

The prefixed design doesn't have any drawbacks as far as I can tell,
and would allow for more intuitively designed blocking/allowing
hierarchies. E.g., if there were 4 different types of proxies in use,
and you only wanted to allow one, you could be more restrictive with
`proxy` and less restrictive with the more precise, suffixed user
agent of the type you are okay with (e.g., `proxy-gopher`).

Emphasis on "more intuitively designed" - I realize you could
technically accomplish this in the current design by simply adding
`proxy` to the mix, but I think the prefix-based organization makes it
clearer and a bit more intuitive.

Warm regards,
Natalie