Style self-links to make them appear only on hover

[rburing]

• Follow-up to Class reference: Add self-links to methods, properties, etc. godot#91537

css_self_links.mp4

On mobile the link appears by tapping on the line of e.g. the method signature (e.g. tapping on the method name).

The implementation is a bit of a hack, but it works. I would have preferred to add an attribute to the self-link to identify it more easily, but I didn't manage to do it with Sphinx (not sure it's possible).

[Calinou]

Tested locally (together with godotengine/godot#91537), it works as expected.

simplescreenrecorder-2024-05-26_20.08.03.mp4

[mhilbrunner]

Thank you! 🙏