Move proteus exercises to separate xml files #342

[rhinopotamus]

The main thing this does is #342.
Sub-things:

• Hopefully this fixes the test build issue (test-build.yml was trying to build the "web" target but the relevant target here is called "html" instead
• I had to move the preview activity xi:include out of the introduction bc you can't nest another division like inside an introduction.
• I put the "proteus" component on the division so that not even the exercises header shows in non-proteus builds.