Add documentation about the project slug

[stig]

Description

Add documentation about the existing pipeline.project.slug pipeline value.

Reasons

We refer to this elsewhere in documentation, cf snippets/orchestration-examples/serial-group.yml#L10. It makes sense to document it in this list of pipeline values.

Content Checklist

Please follow our style when contributing to CircleCI docs. Our style guide is here: https://circleci.com/docs/style/style-guide-overview.

Please take a moment to check through the following items when submitting your PR (this is just a guide so will not be relevant for all PRs) 😸:

• Break up walls of text by adding paragraph breaks.
• Consider if the content could benefit from more structure, such as lists or tables, to make it easier to consume.
• Keep the title between 20 and 70 characters.
• Consider whether the content would benefit from more subsections (h2-h6 headings) to make it easier to consume.
• Check all headings h1-h6 are in sentence case (only first letter is capitalized).
• Is there a "Next steps" section at the end of the page giving the reader a clear path to what to read next?
• Include relevant backlinks to other CircleCI docs/pages.

[BeFunes]

🔴 Please don't merge this. The content of this pipeline value is incorrect for GitHub App pipelines triggered by GitHub OAuth organizations.
Needs https://circleci.atlassian.net/browse/SC-1904

[stig]

🔴 Please don't merge this. The content of this pipeline value is incorrect for GitHub App pipelines triggered by GitHub OAuth organizations. Needs circleci.atlassian.net/browse/SC-1904

Please use the "require change" option on review in this case, so nobody accidentally merges it given the positive review.

[rosieyohannan]

Hey @stig @BeFunes I'm going to close this PR but make a ticket in the docs backlog to address this when ready. We're moving to a new docs site soon so need to clean things up to avoid losing changes and issues. THanks!