Circular dependency introduced by `cdk docs`

[RomainMuller]

The introduction of the dependency aws-cdk-toolkit depends-on aws-cdk-docs created a cycle through aws-cdk-all, due to the presence of integration tests in most L2 construct libraries (they use aws-cdk-toolkit).

We need to break the cycle:

• Short term: take an un-modeled dependency on aws-cdk-docs and gracefully handle inability to load it (rendering an error message inviting the user to install the missing dependency).
• Long term: Break aws-cdk-toolkit into several sub-packages that would allow integration tests to depend only on the subset of features they use.

[rix0rrr]

I believe this is no longer applicable.