[Apps] [Examples] Improve README layout

Summary: Organizes the examples into collapsible sections, and separated into simple and advanced examples.

Test Plan: Plonk README into https://jbt.github.io/markdown-editor/

Reviewers: bytesofman, #bitcoin_abc, Fabien

Reviewed By: #bitcoin_abc, Fabien

Differential Revision: https://reviews.bitcoinabc.org/D14354