docs: use toctree :caption: and move introduction

The canonical way to add a heading to the ToC is to use :caption:.
Do that.

Let's also move the introduction to the top of the document to be
consistent with most other documents.

Cc: Jonathan Corbet <[email protected]>
Cc: Mauro Carvalho Chehab <[email protected]>
Cc: Federico Vaga <[email protected]>
Cc: Alex Shi <[email protected]>
Cc: Yanteng Si <[email protected]>
Cc: Hu Haowen <[email protected]>
Cc: [email protected]
Signed-off-by: Vegard Nossum <[email protected]>
Signed-off-by: Jonathan Corbet <[email protected]>
Link: https://lore.kernel.org/r/[email protected]

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