Add get_child_pages and get_descendant_pages to Confluence (#1630)

* Add get_child_pages and get_descendant_pages to Confluence Cloud and Server (closes #1613)

Adds dedicated methods to retrieve child and descendant pages for a
given content ID, using the /child/page and /descendant/page REST API
endpoints. These filter to pages only, unlike the existing
get_content_children/get_content_descendants which return all content
types including comments and attachments.

* Fix black formatting and flake8 lint errors

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

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Co-authored-by: Claude Opus 4.6 (1M context) <noreply@anthropic.com>