Update pipelines documentation to describe how 'tags', 'variables',
'image', 'before_script', 'script', and 'after_script' can be
supplied at the top level, to be used by any of the runner mappings,
and also overridden by any of the runner mappings.
Also show an example of capturing the custom spack SHA at pipeline
generation time, so all jobs are sure to run with the same version
of spack, as a means to illustrate the $env:VARIABLE_NAME syntax.
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