Added a short explanation of the difference between zeroshot and guid…

[janspoerer]

Added a short explanation of the difference between zeroshot and guided topic modeling to both of the respective documentations so that users immediately know that there are two very similar methods for providing pre-defined topics

What does this PR do?

Cross-reference from the zeroshot doc to the guided doc (and vice versa).

Add a short explainer of the differences between zeroshot and guided.

Fixes #2237

Before submitting

• [x] This PR fixes a typo or improves the docs (if yes, ignore all other checks!).

[MaartenGr]

It seems there is a small linting issue. Could you check?

[janspoerer]

The linter job seems to succeed now :-)

[MaartenGr]

The linter job seems to succeed now :-)

Thanks! Did you see my comment above?