Working with the new documentation CI pipeline has uncovered problems caused by
attempts to cache the entire repo workspace. Initially there are no problems
but as pipelines are testing subsequent iterations of a branch, history may
become non-linear. In one case repo refused to synchronize (with repo sync
--force) the updated repository citing lack of common history.

In the experimental Oniro pipeline I've since switched to keeping a cache of
the git repo mirror, not repo workspace. This also helps with cache hit when
multiple releases are being tested, as cache is mostly static (per release) and
subsequent pipelines do not fight with each other by overwriting the cache to
their preferred state.

Some variable names are updated to better reflect what is going on. Ample new
documentation is added, as code comments, to explain how things work.

Note that some of the defaults are unexpected, as they are coming from the
oniro.git pipeline, and eventually will be included. This can be seen as
variables defined on the .oniro-repo-workspace job that are subsequently
re-defined in the build job.

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