Examples need updated project files

[ascholerChemeketa]

It would be nice if every example had a working project file so that they all can be built with pretext build html, pretest build pdf, or pretext build latex.

Not all examples have project files. e.g. hello-world
Others have ones that are out of date and fail to build. minimal produces:

critical: Failed to parse project.ptx. Please fix the following errors:
error: Field required (('latex',); missing)
error: Field required (('pdflatex',); missing)
error: Field required (('xelatex',); missing)
error: Field required (('latex',); missing)
error: Field required (('pdflatex',); missing)
error: Field required (('xelatex',); missing)

[rbeezer]

Right. See

#1959

#2087

Advertising on pretext-dev might garner some interest?

Extant examples in examples. etc

./doc/guide/project.ptx
./doc/guide/basics/project.ptx
./examples/sample-article/project.ptx
./examples/humanities/project.ptx
./examples/showcase/project.ptx
./examples/minimal/project.ptx
./examples/webwork/sample-chapter/project.ptx