Add validation page

[t-sommer]

fixes #466

[chrbertsch]

Looks really good!

I would suggest the following changes:

• for the online checker, add a link to the sources https://github.com/modelica/fmu-check (see Add link to sources of FMU-Check #281)
• Add section and link for the FMU compliance checker (mentioning the restriction to FMI 1,2)
• for fmpy: describe step-by-step how to use fmpy for validation (or link to special page)
• does fmusim provide diagnostics? then we should mention it

[t-sommer]

@hubertus65, do you want the FMI library listed here?

[chrbertsch]

@hubertus65, do you want the FMI library listed here?

I think you mean the FMU Compliance checker https://github.com/modelica-tools/FMUComplianceChecker (that uses FMI library), which I would list mentioning that it does not check for FMI 3 and currently is not further developed.

[chrbertsch]

I think we should merge this soon and improve later