mention demo_ui as an example in docs

[Brambor]

Summary

None

Purpose of change

• mention demo_ui as an example in docs
• remove nop operation
• add empty lines before lists so that lists compile on my local markdown - they already render fine on GitHub

Describe the solution

Describe alternatives you've considered

Testing

Compiled, demo works.
Compiled the docs locally, HTML looks better locally (lists work).

Additional context

Today I wanted to make a ImGui window. According to docs, I looked at input_context.cpp, but didn't manage to get a window running from nothing. So I copied input_context.cpp and input_context.h and cut it down from 2300 lines (.h and .cpp combined) to about 550 lines. But some basic features weren't working. I recently worked with input_context.cpp, so it probably would take even more time for somebody else.

Then, I discovered #72171, got working version pretty fast. The template has only 67 lines, which is amazing. I decided to share this amazingness with whoever reads the docs next. Now my file is like 200 lines and works better!