fix: Empty Page SubElements

[holtskinner]

• Changed page elements to be initalized with document text instead of page text.
  • Caused all paragraphs/blocks/etc after the first page to be empty.
• Introduced in feat: added hOCR export functionality #123
  • https://github.com/googleapis/python-documentai-toolbox/pull/123/files#diff-af92c6de8f8e84ca66d2fb9fa7e9bddb5bd644e944153bf7a78d35f47c05853eR251
• Added new Unit test which covers the issue
• Discovered in QA DocAI can't work because of NaN's (and little update request) GoogleCloudPlatform/generative-ai#217

[holtskinner]

This issue could have also been part of the reason for #160