ci: add documentation for the continuous integration system

Brand-new documentation for the four repositories: manifest, meta-ohos,
xts_acts and docs is provided in a format that fits the new documentation
project.

I chose to write it from scratch and avoid the extensive graphics
present in the "ci-docs" repository as I wanted to focus on the essence
first. I plan to follow up with documentation of the REUSE and DCO
check jobs. Separately I would like to explore limited diagrams where
they would convey additional benefits.

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