Add index.html to the asf-site branch

[kou]

Describe the enhancement requested

My idea:

• We don't generate the documentation for https://arrow.apache.org/go/
• We just have a simple HTML that have links to https://pkg.go.dev/github.com/apache/arrow-go/arrow and so on in the asf-site branch (that is published to https://arrow.apache.org/go/ )

See also: #139 (comment)

Component(s)

Documentation