jsii-pacmak: H5 tags on Markdown files break Python packaging

### Describe the bug

.md files that have H5 tags (`#####`) and are included to be packaged is breaking packaging with:
`TypeError: Cannot read properties of undefined (reading 'repeat')`

### Expected Behavior

jsii-pacmak completes Python packaging successfully

### Current Behavior

`TypeError: Cannot read properties of undefined (reading 'repeat')`

### Reproduction Steps

I have inline documentation to be generated by `jsii-docgen`. I have no issues when I use H1-H4 header tags in my JsDoc-style comments. But when I use H5 header tags (`#####`), jsii-pacmak fails on Python packaging. This completes successfully for Javascript. Compilation succeeds successfully.

### Possible Solution

Unknown

### Additional Information/Context

_No response_

### SDK version used

5.3.13/1.94.0

### Environment details (OS name and version, etc.)

macOS Sonoma

Start by reproducing the failure in jsii-pacmak using a Markdown file with an H5 heading, then trace how jsii-docgen output is consumed during Python packaging. Compare the successful H1-H4 and JavaScript cases. Done means Python packaging completes successfully for content containing H5 tags.

Written by the indexing model from the issue text.