Update MarkdownReader to parse text before first header

[joelrorseth]

Description

Fixing an issue in MarkdownReader where text that occurs before the first header in a markdown file is ignored. Also fixed an apparent bug where, due to naive concatenation, a trailing newline is added to the end of every text chunk (if this is intentional, please let me know, however no documentation nor tests seem to indicate). I've added tests to clarify these MarkdownReader behaviors and others.

Fixes #13283

New Package?

Did I fill in the tool.llamahub section in the pyproject.toml and provide a detailed README.md for my new integration or package?

• Yes
• No

Version Bump?

Did I bump the version in the pyproject.toml file of the package I am updating? (Except for the llama-index-core package)

• Yes
• No

Type of Change

Please delete options that are not relevant.

• Bug fix (non-breaking change which fixes an issue)

How Has This Been Tested?

Please describe the tests that you ran to verify your changes. Provide instructions so we can reproduce. Please also list any relevant details for your test configuration

• Added new unit/integration tests
• Added new notebook (that tests end-to-end)
• I stared at the code and made sure it makes sense

Suggested Checklist:

• I have performed a self-review of my own code
• I have commented my code, particularly in hard-to-understand areas
• I have made corresponding changes to the documentation
• I have added Google Colab support for the newly added notebooks.
• My changes generate no new warnings
• I have added tests that prove my fix is effective or that my feature works
• New and existing unit tests pass locally with my changes
• I ran make format; make lint to appease the lint gods

[logan-markewich]

Awesome, looks good to me! And the logic is a little more simplified too, nice