refactor: Refactor markdown_to_tups method to better handle multi-lev…

[minglu7]

…el headers and code blocks

Description

• Introduced a dictionary (headers) to track multi-level headers and ensure proper hierarchical structure.
• Improved handling of code blocks by skipping header parsing inside code blocks.
• Added logic to append previous content before switching to a new header level, preserving section context.
• Cleaned up unnecessary headers when a new header of a higher level is encountered.
• Post-processing of content to remove HTML tags and strip whitespace from headers.
• Ensured that empty sections are handled gracefully, with a fallback placeholder for missing content.

Fixes # (issue)

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)
• New feature (non-breaking change which adds functionality)
• Breaking change (fix or feature that would cause existing functionality to not work as expected)
• This change requires a documentation update

How Has This Been Tested?

Your pull-request will likely not be merged unless it is covered by some form of impactful unit testing.

• I added new unit tests to cover this change
• I believe this change is already covered by existing unit tests

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