Refactor extensions docs to enable versioning

[jordojordo]

Summary

Fixes #12357

With the upcoming changes to the extension documentation, the existing docs should be versioned as they pertain to older versions of @rancher/shell and older versions of Rancher that extension developers may still want to write extensions for.

Occurred changes and/or fixed issues

• Added versioning for the Extensions documentation
• Created one version for 2.0.x relating to @rancher/shell version 2.0.x
• Refactored sidebars to fit new structure

Technical notes summary

Areas or cases that should be tested

• Visiting extension docs, internal docs, and blog
• Switch the version for the extension docs

Areas which could experience regressions

Screenshot/Video

Checklist

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• The PR has a reviewer assigned
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[aalves08]

Got a look at this and it works and builds fine. Just one small thing:

1. Can we set next to 3.x.x? Seems the appropriate label, imho

[jordojordo]

We can do that, we'll just have to remember to change the "next" version label whenever we update things past version 3 of course.

[aalves08]

LGTM