docs: add a guide for testing a custom driver plugin locally

[mvanhorn]

What

Adds a Development docs page, Testing a Custom Plugin, covering how to run and test a third-party driver plugin in a local debug build.

Why

In #1747 a plugin author wrote their own driver and hit a code signing error ("Unable to install plug-in") when trying to package and install it like a registry plugin. The maintainer answered with the workflow but it wasn't documented anywhere: the existing Development pages cover building, architecture, and plugin-registry, but none explain how to develop and locally test a brand-new plugin without hitting the signing wall.

Changes

• New page docs/development/testing-plugins.mdx: add the plugin to the Copy Plug-Ins build phase of the TablePro target, Cmd+R to run and test, then remove it from Copy Plug-Ins for a clean release build. Includes a short explanation of why the signing error happens (registry plugins are signed and verified; a locally-built plugin runs under the debug build's signature). Links to the existing Building and Plugin Registry pages rather than duplicating their content.
• docs/docs.json: register the page in the Development group, right after development/plugin-registry.

Docs-only change. No code or CHANGELOG entry per the docs-only rule.

Fixes #1747

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