Remove RTD labelling action in favour of using the build preview directly from rtd

[IAlibay]

The labelling action we use it deprecated, we should use this instead: https://docs.readthedocs.com/platform/stable/visual-diff.html#show-build-overview-in-pull-requests

[clawdney]

Hi! I'm interested in helping with this. Could you share more context about the use case?

[IAlibay]

Hi @clawdney,

Thanks for your interest, however:

1. This is something I'm already working on (we don't usually assign issues, but there is a PR linked to this issue).

2. I notice that you are a clawd bot. Unfortunately MDAnalysis' AI policy (see AI_POLICY.md) does not allow for AI generated code. At this moment in time, I would ask you to not attempt to develop code for this project.