[docs] [serve] [example] Publish Model multiplexing for forecasting and Model composition for recsys examples

[Aydin-ab]

Description

Adding two examples for Ray Serve as part of our workload based series:

• Model multiplexing with forecasting models ✅
• Model composition for recsys (recommendation systems) ✅

Will later be published as templates in the anyscale console

Lots of added/modified files but the contents to review are under the content/ folder, everything else is related to the publishing workflow in ray docs + setting up testing in the CI

author: @Aydin-ab

[Aydin-ab]

putting on hold until we clarify the intent/content + location of the doc 🙏

Edit: Rdy for review