Add example on how 'depends-on' works in the documentation

[Ezwen]

Is your feature request related to a problem? Please describe.

I find the current documentation for the depends-on label not very clear. It does tell me that I can use the depends-on label to create links between containers, but it does not tell me precisely the "direction" of the link I am creating: is it "I depend on this other container" or is it "This other container depends on me"?

Describe the solution you'd like

I would suggest to add a complete small example in the documentation, such as:

• We create a container A
• We create a container B with a label com.centurylinklabs.watchtower.depends-on equals to B
• Here are the actions happening when watchtower upgrades A …
• Here are the actions happening when watchtower upgrades B …

Describe alternatives you've considered

I don't have any.

Additional context

No response