404 Page needed

[ryuuart]

We don't have a 404 page right now. It's using the default one from the Astro Starlight which we don't want. We have to configure it in the Astro Starlight /content/docs/404.mdx. Have to use mdx and layouts because of current limitations in Astro Starlight. Right now it injects the 404 page and collides with the default, global 404 page.

[ryuuart]

withastro/starlight#406 will need to be resolved before working on this. For now, the Starlight 404 page will be used.