Add documentation to files in `com.virtuslab.pulumikotlin.codegen.sdk`

[jplewa]

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Since the files from com.virtuslab.pulumikotlin.codegen.sdk are included in the published library, we need to document them, specifically:

• custom documentation should be added to CustomArgs (based on this),
• same with the overloaded Pulumi.run.

[myhau]

80-90% of the work was done here: https://github.com/VirtuslabRnD/pulumi-kotlin/pull/71, so this task is about making it perfect from the end-user perspective.

We should also focus on code comments that don't bring much value. Comments should probably not explain "what happens" step-by-step (e.g. "registerMapper adds mapper to a list of mappers"). They should answer the "why?" question.