GitLab

build-generic: refactor and document job templates

Split the existing .workspace and .build jobs into the following:

- .workspace - handling git-repo and single-repository CI override
- .bitbake-workspace - handling bitbake customizations, including recipe CI
- .build-{linux,zephyr,freertos} - build a tailored set of recipes
- .build-recipe - build a single recipe
- .build-image - build a single recipe and collect the resulting image
- .build - (deprecated) - alias for .build-recipe

This should keep compatibility with downstream CI (meta-ohos and others)
but allow the docs repository to reuse the .workspace job entirely.

In addition, document all the jobs with reStructured Text, moving some
of the text from the implementation (yaml) files, into new documentation
chapters. Existing documentation is slightly re-structured to take the
new implementation details into account.

In last addition, the .workspace job is no longer strict about presence
of a directory called "sources/". This prevented re-using the job for
gitee builds.

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