Consider renaming the "SDKs" node of the documentation tree

[pcholakov]

Clicking on "SDKs" in the documentation tree does not actually take one to anything related to SDKs; it currently expands to this:

We might want to rename that node to something like "Building with Restate" and create a nested node within it for SDKs, that in turn contains links to dedicated pages for the various SDKs and our CDK constructs.