Target reference: Accurately calculate target weights for traffic distribution

https://support.konghq.com/support/s/article/How-to-accurately-calculate-upstream-target-weights-for-traffic-distribution has really useful info for understanding how Target weights work. We should add this info to the Target entity doc.

Read the linked Kong support article, then locate the Target entity documentation in this repository and compare its current explanation of target weights with the article. Add the relevant guidance for accurately calculating traffic-distribution weights, with the Target documentation clearly reflecting the source material.

Written by the indexing model from the issue text.