Add example of XPath predicate for filtering to do summary sums in repeats

https://forum.getodk.org/t/issue-with-cumulative-sum-inside-a-repeat-group/53460/5?u=ln

Start by reading the linked forum discussion about cumulative sums inside a repeat group to understand the requested XPath predicate example. Add the filtering example to the appropriate documentation page, with enough context to show how it supports summary sums in repeats, and verify that it matches the forum's guidance.

Written by the indexing model from the issue text.