ci: docs synchronization

[piyoki]

Background

Enable docs synchronization on the fly - mentioned in #97

Checklist

• The Pull Request has been fully tested

Full changelog

• ci: run check-docs pipeline ONLY when new PR is raised; do not monitor events such as pushes to the default (main) branch
• ci: add a dedicated workflow (sync-docs) to handle config.dae synchronization
• ci: ensure pipeline should ONLY kick off when new commits are made to the default (main) branch

Issue reference

Implement #97

TODOs

• Add another script to monitor changes to *.md

[piyoki]

Test passed, ready for review.

Successful build cases:

• https://github.com/yqlbu/dae/actions/runs/5099444553
• https://github.com/daeuniverse/dae-docs/pull/25/files

[mzz2017]

Nice job!