Update pipeline-values.adoc

[martincircleci]

Add pipeline parameter pipeline.project.slug to the available pipeline parameter docs

Description

Added the pipeline.project.slug pipeline parameter to the available pipeline parameter docs.

Reasons

Customer reached out to let us know that this parameter is recommended to be used with our serial group functionality, but is not present in our available pipeline parameter docs

Content Checklist

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[BeFunes]

Please let's avoid documenting this pipeline value because it's currently calculated incorrectly for any GitHub App pipeline triggered by a OAuth organization.
Tickets to follow:

• https://circleci.atlassian.net/browse/SC-1903
• https://circleci.atlassian.net/browse/SC-1904