docs(Observer): add documentation to Observer interface

[jakovljevic-mladen]

Description: Since I proposed a TapObserver documentation page PR (#6944), I thought that we should document the Observer interface page as well. Although there are a guide and a glossary pages that explain most of what I wrote in this PR (which I also mentioned in this PR), I wanted to still add the basic documentation to the Observer interface page which users can encounter while roaming the docs app (which is empty for now).

This is how these changes look like:

Related issue (if exists): None