No need for the "web" extra

[vineetbansal]

When I started the project, the idea was to have "core" guidescanpy code and the "web" code on top of it. This is the reason we have the [web] extra available to install, along with its mention in the documentation.

Over time this distinction between "core" code and "web" code has become muddled, and I don't see a big need for maintaining this distinction (since the code makes no clear separation of this, plus the dependencies for the core/web code overlap anyway). Users who don't care about the web component can simply ignore it.

So it would be nice to get rid of the [web] extra and make those dependencies the regular dependencies of guidescanpy, and tweak the documentation at the relevant places.