Gateway JWT Signer doc rework

## Definition of done
The JWT Signer plugin doc is outdated and missing a lot of info that customers need.

We need to:
* rework the [reference/landing page for JWT Signer](https://developer.konghq.com/plugins/jwt-signer/)
* add a use case table and a how it works section
* create a bunch of new examples to cover the use cases mentioned in the draft doc

## Information
@ajessup is working on a revamp of this doc: https://docs.google.com/document/d/1dBOirt-1k0lBXK4KMvki3AMEUX4vKWJoSVizuUmk24o/edit?tab=t.0 - this doc will be the draft that we can base the updates one, when complete.

## Size
L

Start with the JWT Signer reference landing page and the linked Google Docs revamp draft from @ajessup. Use the completed draft to rework the page, add a use case table and how-it-works section, and create examples for the documented use cases. Done means the landing page contains the requested information and examples.

Written by the indexing model from the issue text.