Add custom css to increase content width

e.g. https://github.com/bluesky/ophyd-async/blob/main/docs/conf.py#L296 -> https://blueskyproject.io/ophyd-async/main/tutorials/implementing-devices.html

Would anyone be opposed to this?

Start by comparing the linked ophyd-async docs/conf.py example at line 296 with this template's documentation configuration. Build the documentation locally and verify that the generated content area is wider on a representative page such as implementing-devices.html.

索引モデルが issue の本文から書いたものです。