Add descriptions in SDK docs

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Issue details

It would be helpful to add descriptions to the packages, interfaces, and classes.

Packages are particularly bare, so a sentence on what the package is about would be helpful for navigating the docs. It would also be helpful to note when a package is not really meant for external consumption in Pulumi programs (.internal implies this, but codegen is also probably not for use by end users either):

Coverage in classes is better, but still not complete:

Adding linting would be cool if it's easy enough.

Affected area/feature

Start by reviewing the SDK's packages, interfaces, and classes to identify undocumented or incomplete descriptions. Add concise descriptions that explain each package's purpose and indicate packages not intended for external Pulumi use; consider linting only if the existing documentation tooling supports it. Done means the affected SDK documentation has useful coverage across these areas.