Aggregate manifest repository documentation

The manifest repository now contains standalone documentation which is
aggregated under `repos/manifest`. The section about testing in
`ci/manifest.rst` is replaced with a short reference. This effectively
completes the move of CI documentation from the docs repository to the
manifest repository.

This leaves an open question on how to handle the meta-ohos repository,
which also contains general build system documentation. As a likely
special case, this is not discouraging and I would like to keep adding
other repositories into the `repos/` structure.

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