Use solph instead of oemof in the documentation

[lensum]

Description

The aim of this PR is to consistently refer to solph instead of oemof in the documentation. I seem to remember from the 2022.11 meeting, that oemof should no longer be used synonymously with solph. However, in the documentation sometimes oemof is used while actually referring just to the solph package. A prominent example is the title of the documentation (file docs/index.rst), where it reads "Welcome to oemof's documentation!".

As oemof is a framework containing multiple packages by now, I think it would be better to be precise on which part of oemof is meant.

Questions

• Which style would be best to use in regular text? I can think of:
  • solph
    • it's the name of the package, after all
  • oemof.solph
    • biggest resemblance to the in-code use
  • oemof-solph
    • also used in the documentation from time to time
    • best solution for bringing the relation between oemof and solph across (imho)

[p-snft]

Thanks a lot. What comes on top is that the package is actually called oemof.solph but sometimes the fullstop is not allowed. So, we have to mix oemof.solph and oemof-solph as an alternative.

I already approve as this is already a lot better than before.