Use examples' README front matter to generate the demo website

[abey79]

Currently, a list of of examples and related metadata is hardcoded in script/build_demo_app.py, but this is somewhat redundant with the examples' README front matter. I suggest improving this script such that:

• it pulls all example whose front matter includes demo: true
• it pulls the metadata from the front matter

Examples' front matter will need to include:

• demo: true (if the example is to be included as demo)
• description: ... (needed anyways for the example landing page)
• demo_build_args: [] for optional build argument when generating the demo site

Some mechanism should also be devised to generate/avoid script/requirements-web-demo.txt.

[jprochazk]

This was done in #4418 - the build_examples and build_examples_manifest binaries don't hardcore a list of examples anymore, everything is read from the filesystem.