Explain DevOps "index score" interpretation

What does this MR do?

Adds a sentence to help explain the meaning/interpretation of the "index score" value in the DevOps Score feature.

This is motivated by an internal conversation in Slack#s_manage between @djensen and @laurie_howitt, where Laurie mentioned: "the documentation doesn’t really describe how to interpret the score, how does one interpret it?"

Does this MR meet the acceptance criteria?

Conformity

• Changelog entry
• Documentation (if required)
• Code review guidelines
• Merge request performance guidelines
• Style guides
• Database guides
• Separation of EE specific content