DOC Add installation instruction to docs

[BenjaminBossan]

I think the docs should have an installation instruction. Thus I added a new section there but LMK if I should put it elsewhere instead @skops-dev/maintainers

[adrinjalali]

I'm not sure why you prefer this to pip install skops.

[BenjaminBossan]

This the recommended way in the docs. The reason I've seen in the past is that even though in general it's the same, in some situations pip install can lead to the package being installed globally, instead of the specific python environment. In contrast, python -m pip install will never fail where pip install works.