Add export/import functionality for documentation

[Copilot]

Users needed ability to export documentation for backup and re-import to address backend display issues or migrate between sites.

Changes

Backend

• Added ExportImport class handling JSON export/import via AJAX
• Export captures complete doc structure: content, hierarchy, metadata (votes, contributors), tags, ordering
• Import preserves parent-child relationships via ID mapping, skips existing docs by slug
• Nonce verification and manage_options capability gating on both operations

Frontend

• React component at #/export-import with separate export/import UIs
• Export triggers JSON download with timestamp
• Import validates JSON file, displays result summary (imported/skipped/errors)

Security

• Input sanitization: wp_kses_post for content, sanitize_text_field for strings, absint for integers
• Type-specific meta sanitization (votes as int, contributors as int array)
• Tags and taxonomy terms sanitized during import

Data Structure

{
  "version": "2.1.18",
  "export_date": "2024-01-31 12:30:45",
  "docs": [{
    "id": 123,
    "title": "Doc Title",
    "parent": 0,
    "meta": {"positive": 5, "negative": 2},
    "tags": ["tag1", "tag2"]
  }],
  "tags": [{"name": "Tag", "slug": "tag", "description": "..."}]
}

Menu location: weDocs → Export/Import

Original prompt

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This section details on the original issue you should resolve

<issue_title>[Feature] Export all docs to move to another page / re-import them</issue_title>
<issue_description>I'm currently facing the problem that still not all docs are shown in the backend, but in the front end. I thought about to export all those pages and re-import them. But I did not find this option in the plugin. Any chances this is considered?</issue_description>

Comments on the Issue (you are @copilot in this section)

• Fixes [Feature] Export all docs to move to another page / re-import them #243

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