feat: "categories" should be able to have content

[Hunam6]

This config

"sidebar": [
	["index.mdx", "/"],
	["page-at-root.mdx", "/page-at-root"],
	[
		["part-one/index.mdx", "/part-one"], // this isn't allowed currently
		[
			["part-one/abc.mdx", "/part-one/abc"]
		]
	]
]

should make those pages available: /, /page-at-root, /part-one and /part-one/abc.

---

Right now the best equivalent is this

"sidebar": [
	["index.mdx", "/"],
	["page-at-root.mdx", "/page-at-root"],
	[
		"part-one",
		[
			["part-one/index.mdx", "/part-one"],
			["part-one/abc.mdx", "/part-one/abc"]
		]
	]
]

Which doesn't really work and even creates a bug when we click on "part-one/abc.mdx":