Update tracing documentation

What does this MR do and why?

Update tracing documentation

• Update documentation to reference project access token instead of group access token (more natural since tracing is project level)
• Align with guidance provided in the following comment gitlab-org/opstrace/opstrace#2426 (comment 1600691071)

Closes gitlab-org/opstrace/opstrace#2426 (closed)

Closes gitlab-org/opstrace/opstrace#2432 (closed)

Signed-off-by: Mat Appelman mappelman@gitlab.com

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