Improved documentation (examples, exported APIs, playbooks, etc.)

Hi everyone,

The documentation for the current repo could benefit from more explicit guidance on how to use the APIs. In general, the repo is solid, but it is hard for "outsiders" to understand its full capabilities, which hinders its adaptability, accessibility, and usability.

Here are some examples:
- In the getting started documentation, there is $alpha$ vs $\alpha$.
- The README.md should include examples on how to instantiate Tableaux, SAT solvers, etc. Ideally, people should understand the repo's full capabilities (e.g., different solvers, formulas, and semantics).
- What exactly is exported?
- How should one use this repo?
- What are the known limitations?
- Are there playbooks/playgrounds for users?
- Etc.

These are just a few examples. Some of them are general observations that could be applied to other repos as well within the `Sole.jl` ecosystem.