(GH-612) added a section on non-local build providers...

[nils-a]

This is currently more a base for discussion.

I started out to make it more clear, that - if multiple builds are made - one buildprovider/os combination has to be "primary". Twoards that end, I added a section on "CI-systems" (i.e. non-local builds)

I sorted the section between Usage and Extending because

• a) I felt that the top sections (Overview - Extending) are about "using" Cake.Recipe and the lower sections are more "other" stuff.
• b) I felt that having multiple builds was more common than modifying builtin tasks 😊

Things I am uncertain about:

• I don't like the title Overview in the overview.md but I had no better idea. Since the list of providers will never be really long, maybe could skip the overview page and add the more general informations direcly into index.cshtml.
• I don't like the word "specifics" but I lack a better translation for the thing in my head. (Maybe 'idiosyncrasy'?) But then, the whole section is not necessary because the goal would be that all systems behave the same, right? (So the only specific (idiosyncrasy) would be that gh-actions currently lacks the ability to auto-upload the artifacts.)
• On one hand I like the idea of having the build-yaml here for easy an start... On the other hand they are not really Cake.Recipe specific are they?

[gep13]

Just a few small suggestions!

[gep13]

@nils-a this is AMAZING! Thank you so much for pointing all this together, I really appreciate it! I left some comments for a few suggestions.

[nils-a]

@gep13 I included your comments. I am still testing the example-builds provided here.

[nils-a]

@gep13 @AdmiringWorm I have added the travis-example and made some modifications. Could you please have a renewd look at it? (And I promise, when we're done I'll squash the commit-jumble I have created here...)

[gep13]

LGTM!

[gep13]

LGTM!