Feat: Detect all text in HTML Heading tags as titles (#1556)
## Summary
This will increase the accuracy of hierarchies in HTML documents and
provide more accurate element categorization. If text is in an HTML
heading tag and is not a list item, address categorize it as a title.
## Testing
```
from unstructured.partition.html import partition_html
elements = partition_html(url="https://www.eda.gov/grants/2015")
```
Before, the date headers at the given url would not be correctly parsed
as titles, after this change they are now correctly identified.
A unit test to verify the functionality has been added:
`test_html_partition::test_html_heading_title_detection` that includes
values that were previously detected as narrative text and uncategorized
text