Add pyvespa notebooks

[kkraune]

This PR:

• links to all pyvespa sample apps in notebooks
• categorizes the pyvespa sample apps

Later PRs:

• integrate the "regular" samples with the pyvespa ones, using these categories - possibly add/change the categories
• add a short intro to the pyvespa apps, and reorder apps within category for best flow. Make the categories coherent

Motivation:

• this will find its way into getting started docs and vespa consoles - with the goal of helping the users find relevant examples for their use case. Using this readme as a source of truth for now (all samples are now linked from here)

[kkraune]

With this, we have added info for all sample apps. next step is to merge into fewer units and improve the flow. possibly remove a sample app or two. Merging for now