The JSON export: Support composable documents

What's Changed

• Bump version to 0.0.52 by @stanislaw in #1738
• export/html: collapsible_tree.js: double-click only affects current and child folders by @mettta in #1739
• export/json: handle more corner cases: nodes without levels, included documents by @stanislaw in #1740
• document_iterator: refactor the iteration algorithm by @stanislaw in #1742
• Code climate: improve a grammar form object class name by @stanislaw in #1743
• Remove all BibTeX bibliography-related code and pybtex dependency by @stanislaw in #1744
• docs: update requirements (ZEP, data integrity, fix all missing statuses, a few rationales) by @stanislaw in #1745
• export/json: do not export included documents by default by @stanislaw in #1746
• docs: document less portable features by @stanislaw in #1747
• export/html: update 'legend' markup on search/git screens by @mettta in #1748
• export/html: add zoomable.js: to zoom and move DTR screen by @mettta in #1741
• export/json: fix and simplify the iteration algorithm one more time by @stanislaw in #1749
• docs: update Roadmap by @stanislaw in #1751
• Code climate: CompositeRequirement -> SDocCompositeNode by @stanislaw in #1750

Full Changelog: 0.0.52...0.0.53