Add experimental build pipeline

This pipeline allows aggregation content from other repositories that
participate in our development manifest, performs a local test build
and if successful, exports the content into a new "aggregated docs"
repository at https://git.ostc-eu.org/OSTC/infrastructure/openharmony-readthedocs-aggregated



That repository is compatible with readthedocs build system and only
needs a webhook to trigger automatic builds that show up on the correct
readthedocs subdomain. The webhook needs to be set up separately by
whoever owns the "openharmony" project on readthedocs.

Signed-off-by: Zygmunt Krynicki <zygmunt.krynicki@huawei.com>