docs(core): plugin section

[isaacplmann]

Adds a new plugin section for all documentation related to creating plugins, executors and generators.

• The plugin registry is moved into this new section
• The community page doesn't exist for the moment since without the plugin registry, it's just the social links which are in the top right on every page. Later we'll reintroduce the community page with the Nx Champions section.
• Once the node CLI feature is ready, I'll update the Create a Preset guide to include instruction on setting up a create-my-framework CLI

I also fixed the internal link checker script to check url="/something" links inside of cards.

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Name	Status	Preview	Comments	Updated (UTC)
nx-dev	✅ Ready (Inspect)	Visit Preview	💬 Add feedback	May 5, 2023 4:06pm

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