Add Jira backlog agent documentation and index entries

[peteroden]

Summary

Create docs/agents/jira-backlog/README.md parallel to existing backlog agent docs and index it from docs/agents/README.md.

Why this was deferred

This work came out of the remaining review comments on PR 978. During implementation, PR 978 stayed narrowly scoped to the directly actionable configuration, catalog, skill, and installer fixes. The broader Jira backlog documentation additions is to land in a follow-up PR instead of expanding the current one.

[katriendg]

Closing this one in favor of the more extensive #1189 issue. This one becomes a duplicate.