[Documentation] Updates to NVIDIA Playground/Foundation Model naming.… (#14770)

…  (#14723)

- **Description:** Minor updates per marketing requests. Namely, name
decisions (AI Foundation Models / AI Playground)
  - **Tag maintainer:** @hinthornw 

Do want to pass around the PR for a bit and ask a few more marketing
questions before merge, but just want to make sure I'm not working in a
vacuum. No major changes to code functionality intended; the PR should
be for documentation and only minor tweaks.

Note: QA model is a bit borked across staging/prod right now. Relevant
teams have been informed and are looking into it, and I'm placeholdered
the response to that of a working version in the notebook.

Co-authored-by: Vadim Kudlay <[email protected]>