DOC - extend the documentation of the base Selector class with examples on composing selectors and general usage

[rcap107]

Describe the issue linked to the documentation

I asked an agent to compose selectors to choose specific columns in a dataframe, and it immediately looked for the documentation of the base Selector class for information on this. The Selector docstring is very terse, and does not explain at all the basic features of the selectors.

Suggest a potential alternative/fix

There should be short but general examples of how selectors in general work in the docstring of the base class.

[omkar-334]

hi @rcap107 can i work on this?

[jeromedockes]

thanks @omkar-334 but @rcap107 is currently exploring various possible adjustments to the doc and packaging around this topic, so it is best if we wait until we have a clearer picture