[JOSS revision] Improve INSTALL.md and documentation

[cvanaret]

Revision based on openjournals/joss-reviews#10229 (comment)

Major action points:

• Installation. I'd be nice to have more streamlined installation options. I'd move the precompiled/executable instructions to the top of INSTALL.md. I'd also streamline the use of unopy for Python users.
• Documentation. Currently, installation instructions, instructions on using the different interfaces, case-studies/examples and theory are scattered across multiple *.md files cited in the main README.md in the project's root folder as well as in https://unosolver.readthedocs.io/en/latest/. I'd recommend unifying all of this information in a single readthedocs webpage. This will centralize installation instructions, theory, examples and so on. You already have everything here, it's just about reorganizing information. I've found a bit confusing to navigate throughout several *.md files and the page to understand all aspects of Uno.
• OS specific instructions. The "Precompiled libraries and executables" section on INSTALL.md gives somewhat generic/somewhat linux-ish instructions. I'd recommend having OS-specific sections and move this to the top of this page (per comment 1 above).

Minor action points:

• While restructuring your documentation, I have a minor suggestion: Having a section quickly showing uno through the interfaces. Something like a "Getting Started" or "Usage". One example can be seen here.. Maybe showing unopy + other interfaces in action. I was able to run example_hs015.py successfully. Maybe this is a good candidate?

[cvanaret]

@amontoison can you have a look at:

• the result in the joss_revisions branch
• the doc

when you have a moment?

I created symlinks from the READMEs to docs/interfaces for C, Fortran and AMPL. Julia and Python have their own READMEs and doc pages.

I need to refine the "Getting started" page before merging.