Added further details to the CI_PROJECT_DIR variable documentation

What does this MR do?

Provides further clarification to the CI Variable CI_PROJECT_DIR as there is an undocumented dependency to the GitLab Runner builds_dir configuration parameter. If the runner config option is set it will use this to build a path.

For example:

Without the builds_dir option set:

$ & echo "${CI_PROJECT_DIR}
C:/Program Files/GitLab-Runner/builds/183c56d5/0/mygroup/myproject

With builds_dir set to "/builds":

$ & echo "${CI_PROJECT_DIR}
/builds/183c56d5/0/mygroup/myproject

Related issues

None.

Author's checklist

• Apply the correct labels and milestone
• Crosslink the document from the higher-level index
• Crosslink the document from other subject-related docs
• Feature moving tiers? Make sure the change is also reflected in features.yml
• Correctly apply the product badges and tiers
• Port the MR to EE (or backport from CE): always recommended, required when the ee-compat-check job fails

Review checklist

• Your team's review (required)
• PM's review (recommended, but not a blocker)
• Technical writer's review (required)
• Merge the EE-MR first, CE-MR afterwards