Remove pre-1.0 documentation from the agones.dev website

[roberthbailey]

Is your feature request related to a problem? Please describe.
Right now agones.dev has documentation for releases going back a long time, to pre-1.0 version. The drop down shows:

Describe the solution you'd like
We should drop all of the version prior to 1.0. We may also want to drop later versions (see #1146) but we will leave that discussion to a separate issue.

[markmandel]

Makes sense to me! 0.12.0 came out in 2019, so I think we can safely drop it.

[roberthbailey]

On the PR, @markmandel said:

At some point we should also remove the corresponding services from App Engine, but we can do that at a later date.

I've removed the services for 0-9-0, 0-10-0, 0-11-0, and 0-12-0.