Enable building RDKit stubs as part of the conda-forge recipe starting from Release 2023_09_6

[ptosco]

This PR contributes a couple of small changes to enable building RDKit stubs as part of the conda-forge recipe. The stubs are correctly build and included in the tar.bz2 artifact on all supported platforms.

Please note that for the build to succeed on Windows, a small fix is also needed on the RDKit side, as documented in this PR: https://github.com/rdkit/rdkit/pull/7191

PR #7191 has the 2023_09_6 milestone, so it is expected that the conda-forge builds for the 2023_09_6 RDKit release should contain RDKit stubs on all platforms if this PR is accepted.

• modified build recipes to enable building rdkit-stubs
• fixed typo and added comments

[conda-forge-webservices[bot]]

Hi! This is the friendly automated conda-forge-linting service.

I just wanted to let you know that I linted all conda-recipes in your PR (recipe) and found it was in an excellent condition.