fix: add newline rules to separate multiple images in markdown

[Tomic-Riedel]

• Added two new rules in DocumentMarkdownDecoder to handle formatting for multiple images:
  1. Ensures a double newline (\n\n) is added when a single newline separates two images.
  2. Ensures a double newline (\n\n) is added when no newline separates two images.
• Added a test case to verify that multiple images on separate lines without blank lines are properly decoded into distinct nodes.
• Added a test case to verify that multiple images on the same line (inline) are properly decoded into distinct nodes.

This pull request fixes #1113.