ci: prototype new CI infrastructure

The documentation repository relies on a series of symbolic links that
connect it to the oniro.git repository, when used inside a checked-out
repo workspace.

Ever since we switched to the symlink-based override that supports
branch selection for merge requests and merge trains, the docs
repository stopped to build correctly.

This is caused by now-broken symbolic links that once traversed, no
longer point to the files in oniro.git, due to the way the redirection
code works.

An entirely new pipeline was created to avoid this problem, and to unify
other repo-hacking systems that we introduced over time. A prototype of
this new pipeline is now available here, in the docs repository.

The main difference is that local manifests are now used to edit the
repo composition before "repo sync" is used. This allows us to achieve
the same effect we had before, but using first-party systems. The
secondary difference is that the docs pipeline is now separated from
changes (including breaking changes) in the oniro.git pipeline.

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