Update CI structure

[UebelAndre]

Right now we have CI jobs that build and test the core rules and docs and examples in the same job. This may be nice from the perspective of resource management in CI but means that we have very coupled test cases and can end up in situations where examples have crossed boundaries into integration tests (#789) and are no longer useful as examples users can look at to figure out what to do or we introduce a coupling with an aspect and cannot build without a particular configuration (#824 (review)).

I'd propose having the following structure for CI

Job	Runners	Builds
Buildifier	(wherever that runs now)	1
Build and test //... with no configurations	macos rbe_ubuntu1604 ubuntu2004 windows	4
Build and test //... clang with no configurations	ubuntu2004	1
Build and test //... min Bazel version with no configurations	ubuntu1804	1
Build and test //... with all aspects	macos rbe_ubuntu1604 ubuntu2004 windows	4
Test clippy failures	ubuntu2004	1
Test rustfmt failures	ubuntu2004	1
Build and test examples with all aspects	macos rbe_ubuntu1604 ubuntu2004 windows	4
Build docs	ubuntu2004	1
Build crate_universe examples	macos rbe_ubuntu1604 ubuntu2004 windows	4

Total Jobs: 15 -> 22

edit: readability
ps: If it's not obvious, //... would continue to not include examples and docs. We would re-write docs/WORKSPACE.bazel and examples/WORKSPACE.bazel to be normal looking WORKSPACE files (not all just call wrapper macros).

[UebelAndre]

Would love your thoughts @illicitonion @dfreese @hlopko

[UebelAndre]

#828 would probably also be added as another set of tests

Job	Runners	Builds
Build and test //... with no configurations on min rust version`	macos rbe_ubuntu1604 ubuntu2004 windows	4

Total Jobs: 15 -> 26

[UebelAndre]

I think the only thing left to do here is add something which tests a pinned version of Rust. Not quite sure how to implement a toggle such that a particular CI job could define this.

[UebelAndre]

This was completed by #1720