Recommend single Solution target

[nojaf]

To fix my problem in #243, I concluded that running everything in a single pass is the most convenient option. You can do that with a single solution target. FSharp.Analyzers.Build may have been overly complicated to start with. I updated the documentation and am proposing this simpler approach.

Let me know what you think.

[TheAngryByrd]

While I do agree we should propose the simplest options up front, I don't think we can necessarily get away from needing to merge in some circumstances.

Also, moving to a Directory.Solution.targets means doing analysis on individual project can be tedious because it doesn't auto propagate to individual projects like a Directory.Build.targets does.

So while it may solve 1 CI specific issue, I think it causes bigger projects to not want to run analyzers locally if the solution is the only option documented.