Add BUILD_PDF flag

[manubot]

Add BUILD_PDF flag

[dhimmel]

For now, I'd just comment out the PDF section of build.sh. In the future, I agree we should add flags or env variables for all the different outputs types.

However, I am thinking that we should migrate more functionality to the Python package. Specifically, we'd have multiple manubot commands

• manubot prepare (what the manubot CLI currently does)
• manubot output which would run pandoc and could accept --html, --latex, --jats, --docx flags
• manubot webpage which would do what's in webpage.py currently
• manubot timestamp to timestamp outputs / webpage stuff
• manubot deploy which would be called by CI to deploy

Still lot's of thinking to do, but given these plans I think it makes sense to hold off on a BUILD_PDF env variable for the time being.

[dhimmel]

#145 added support for a BUILD_PDF environment variable. Unlike BUILD_DOCX which must explicitly be set to true, not setting BUILD_PDF=false causes it to default to true.