doc: css: fix styling of signatures

[kartben]

https://builds.zephyrproject.io/zephyr/pr/80023/docs/contribute/documentation/guidelines.html#custom-sphinx-roles-and-directives vs. https://docs.zephyrproject.org/latest/contribute/documentation/guidelines.html#custom-sphinx-roles-and-directives

Also impacts definition lists in general, as in it changes their L&F ever so slightly, ex.
https://builds.zephyrproject.io/zephyr/pr/80023/docs/contribute/index.html vs. https://docs.zephyrproject.org/latest/contribute/index.html

Or https://builds.zephyrproject.io/zephyr/pr/80023/docs/services/llext/api.html vs. https://docs.zephyrproject.org/latest/services/llext/api.html

Sync the section of the custom.css dedicated to object signatures with Godot custom CSS.

Fixes #80005.

[pdgendt]

Don't know css, but looks good!