Cleaning up document structure and duplicate entries

[galeep]

As mentioned in #329: this first change in a series aims to enhance the maintainability, consistency, and overall usability of the list. I'm submitting these incrementally to manage the changeset size and minimize drift.

This PR addresses:

1. Heading Consistency: I rectified inconsistencies in about 18 out of 300+ entries by changing the heading level from H5 to H4. This improves readability and parsability.
2. Duplicate Resolution: I merged duplicate country and person entries in the same section. These changes focus on structural consistency in the document. Any entries contributed under different country categories will remain as they are for now.
3. ToC Updates: I have regenerated the table of contents based on the regions specified in the document.

These changes primarily target elements causing parsing difficulties with semi-structured data.

Next steps:

Alphabetization by last name: the upcoming PRs will address alphabetization issues, as per #329
Automation: after addressing alphabetizing, I'll provide the scripts to keep listings alphabetized and the ToC accurate and up-to-date.

P.S. On a technical note, I've introduced a marker (<!-- toc -->) for automated ToC generation. This change will not disrupt any existing ToC generation using the marker that was already present.

[galeep]

Cleaned up an orphaned H4 entry for Japan in Organizers section, leaving the associated organizer's entry in Asia.
(Based on the comment and change in #316, the entry may need to be updated to reflect current info.)

[jlooper]

thank you!