Website: Add PyIceberg, IcebergRust, and IcebergGo to top nav bar

[petern48]

Closes #12910

[manuzhang]

1. We also need to remove their links from java docs
2. We'd better have some form of pull-down list of websites. Otherwise, the nav bar will be too long.

[petern48]

I experimented with trying to achieve what the Spark docs do (as you requested here) by nesting the docs inside a new nav bar field using the below change, and it does NOT create a dropdown. Instead, it just sends you directly to the PyIceberg website (the first entry), making the rest of the links inaccessible. I did some research, and it seems not all mkdoc themes support dropdowns, unfortunately.

nav:
  ...
  - Other:
    - PyIceberg: https://py.iceberg.apache.org/
    - IcebergRust: https://rust.iceberg.apache.org/
    - IcebergGo: https://pkg.go.dev/github.com/apache/iceberg-go/

Instead, I propose that we instead nest them under the existing Docs nav bar entry in the following way. Here's what the new code changes looks like.

Expanding the "Iceberg" section, we can then see what was previously in the Docs nav bar item.

When you originally click on Docs at the top, it defaults to the first page entry (Iceberg -> nightly -> Introduction) as before. What do you think @manuzhang?

[manuzhang]

@petern48 Thanks, I've opened a discussion on the dev list for a more comprehensive restructure of the side navigation. Let's see what others think.