Add Confluence converter for Cloud pages and Storage Format XML

[saheersk]

Introduces ConfluenceConverter (URL-based, strips nav chrome, focuses on
main content) and ConfluenceStorageConverter (XML storage format, translates
ac: macros to Markdown: code blocks, info/note/warning panels, task lists,
status badges, Jira keys, links, and images). Includes test fixtures and
24 unit tests.

#1715

[saheersk]

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