docs: create docs site at `decorators.deno.dev`

[nberlette]

Eventually it'd be nice to have a reliable source of documentation / references for decorators, also serving as a shameless plug to these packages. I already have decorators.deno.dev, but the site needs to be completed and needs an overhaul.

The goals:

• Interactive documentation
• Code snippets that are runnable inline
• Playground specifically for decorators
• Docs for Stage 3 and Legacy Decorators
• twoslash + shiki for code snippets to provide rich syntax annotations
• dynamic og images
• what else?