Add CI_PROJECT_DESCRIPTION to predefined variables

What does this MR do and why?

Closes CI environment variable for project description (#339065 - closed)

This exposes project.description in a CI_PROJECT_DESCRIPTION predefined variable in CI jobs, similar to CI_PROJECT_TITLE added a few years ago.
Some static site generators can use this to generate config when using GitLab Pages without duplicating the project configuration.
For example, MkDocs uses site_description to add a meta tag to the generated HTML header. https://www.mkdocs.org/user-guide/configuration/#site_description

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How to set up and validate locally

Numbered steps to set up and validate the change are strongly suggested.

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