MIPbook should only run from main branch script

[apenzk]

Currently every branch triggers their own script and overwrites the MIPbook.

However branches will have outdated scripts and thus there will be inconsistency in the version of the MIPbook that is displayed.

Suggested improvements

• Version 1: Run a cron job (e.g. every hour) for main. Updates are not reflected immediately. This is ok because we can still test the MIPbook in mip-testing.
• Version 2: Trigger with any commit an update but do so using the script from the main branch.