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 <corbet@lwn.net>
Cc: Mauro Carvalho Chehab <mchehab@kernel.org>
Cc: Federico Vaga <federico.vaga@vaga.pv.it>
Cc: Alex Shi <alexs@kernel.org>
Cc: Yanteng Si <siyanteng@loongson.cn>
Cc: Hu Haowen <src.res.211@gmail.com>
Cc: workflows@vger.kernel.org
Signed-off-by: Vegard Nossum <vegard.nossum@oracle.com>
Signed-off-by: Jonathan Corbet <corbet@lwn.net>
Link: https://lore.kernel.org/r/20231027081830.195056-11-vegard.nossum@oracle.com