Figure out and document the size limit of the CI yaml config

We realized we haven't documented the size limit of the YAML config. According to the code, it's 1 megabyte, but that's only after it's loaded into ruby, which does not directly correlate to the lines/characters in the actual config file:

• https://gitlab.com/gitlab-org/gitlab/-/blob/4525eab3c6a567a9db5acd995eb2e2734ab7f51e/lib/gitlab/config/loader/yaml.rb#L12
• This class determines the loaded size: https://gitlab.com/gitlab-org/gitlab/-/blob/4525eab3c6a567a9db5acd995eb2e2734ab7f51e/lib/gitlab/utils/deep_size.rb

We should find out a reasonable limit that we can add to the docs.