Recently, the RunAsUserName feature has been added, allowing users to
run Windows containers with custom usernames, different from the image
default usernames.
Adds documentation regarding its prerequisites, feature gate, usage,
restrictions, and samples.
This PR deals with the getting started guides (windows) and tutorials
sections. Since the YAML files in the windows directory currently are not
referenced at all, this PR refactored the markdown file to correct this
problem. When appropriate, we use the YAML content from the markdown in
the extracted version.