Documentation: Add basic description on how to run Goblint and on need for context widening

[jerhard]

In #206 (comment), @vogler noted that stack overflows that occur in calls for C-functions result in Goblint being killed with signal 11.

This PR adds a very basic documentation on how to run Goblint, with a remark to enable exp.widen-context when the analysis of recursive function leads to a stack overflow / signal 11.

Related to #208.

[michael-schwarz]

Should we also explain the Makefile support here? Or is this best saved for a different page?

[sim642]

Should we also explain the Makefile support here? Or is this best saved for a different page?

I think this page would be fine for that until it grows too big. Although often these Read the Docs style sites have quite long pages, but with sensible heading structure, the sidebar makes even long pages navigable.

Same goes for these common options/configurations explanations, e.g. exp.widen-context in this case: in the future if we already more of them documented, then we might have to move them to a separate page.

[jerhard]

Should we also explain the Makefile support here? Or is this best saved for a different page?

We can put that into the same file, yes. It would need an a little bit more extensive explanation though, because it e.g. requires having a setup where the cilly executable is in the PATH – which it is not by default, IIRC. So I would move documenting this to another PR/Issue then.