Create new CI and Release workflows.

[fiseni]

Closes #349. Please check the related issue for more details.
Removed the old workflows and created two new ones. I think the workflows now are stupid simple :)

• ci.yml - Build and test, triggered on pr and push on main. Triggered only if there are changes in the library projects. All other changes for docs or sample apps won't trigger anything. It's tedious if CI is triggered on each doc change.
• release.yml - Triggered on release publish. We're anyway always creating releases from Github, so let's use that event. This will publish all three packages (but that's what we're always doing). If in the future we need to publish them independently, we might use different tags for each package, but for now, that seems unnecessary.

The solution contains sample apps too, but I didn't want to build them as part of the workflows. So, instead of passing all the required projects as an argument to dotnet build, I created a solution filter ci.slnf which contains only the necessary projects. I think this was a great trick, and makes the yml files way cleaner.

[ardalis]

Sounds great; let's see them in action!

[ardalis]

Also, nice use of a solution filter; I probably would have created a separate ci.sln file to do the same thing, but your approach is cleaner I think.