Add more complex examples

[bog-dan-ro]

The existing examples are great to get started with simple projects, but are not enough for more complex projects:

• cmake/autotools/meson projects with multiple targets (e.g mpg123, vorbis, etc.).
• According to https://docs.conan.io/en/latest/migrating_to_2.0/recipes.html#the-scm-attribute the scm attribute was removed but Git seems to be missing a few features like "submodule": "shallow", "shallow": True, scm attributes. Again, a more complex example will be more than welcome.

[jngrb]

I agree. Further best-practices example that I would like to see are these:

• Using export and source (instead of the old scm attribute) but where the conanfile (and the CMakeLists.txt) are not in the root of the Git repo.
  I.e. the existing multi-subprojects example conanfile/layout/multiple_subprojects but just with sources from Git.

• Using export and source (instead of the old scm attribute) but where the source files are distributed across several Git submodules.