Restructure JVM docs.

[benjyw]

Previously the top-level menu was called
"Java and Scala" and "Kotlin" was a subpage.

Now the top-level is called "JVM" and there
are two subpages - "Java and Scala", and "Kotlin".

[huonw]

Sensible rearrangement!

I think this rename will mean existing links to https://www.pantsbuild.org/2.19/docs/java-and-scala and https://www.pantsbuild.org/2.19/docs/java-and-scala/kotlin will be broken, so it may be best to either:

• provide old pages to redirect (either here or in https://github.com/pantsbuild/pantsbuild.org/blob/main/old_site_redirects.js somehow)
• not cherry pick this to 2.19 (either deploying with the 2.19.1 release or before it). (It seems fine to break the pre-release links like https://www.pantsbuild.org/2.20/docs/java-and-scala.)

[thejcannon]

FWIW let's use a new file for new redirects. I'm hoping to keep that file strictly old -> new (simply for separations sake)

[benjyw]

Good idea re the redirects. So, how do I preview all this? Once I know how I will document it somewhere central.

[thejcannon]

Generally, I would make edits on pantsbuild.org repo, then you can iterate. When you feel good about it, I'd cp back to this repo and PR

[benjyw]

I have a redirects PR ready in pantsbuild/pantsbuild.org. IIUC, we can merge it once this is merged, since the redirecting files will only be created when we publish.

[WorkerPants]

I tried to automatically cherry-pick this change back to each relevant milestone, so that it is available in those older releases of Pants.

✔️ 2.19.x

Successfully opened #20565.

✔️ 2.20.x

Successfully opened #20564.

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