**Apache Airflow version**: 2.0.2

**What happened**:

Tried to use the following markdown as a `doc_md` string passed to a DAG

```markdown
- Example
    -  Nested List
```

It was rendered in the web UI as a single list with no nesting or indentation.

**What you expected to happen**:

I expected the list to display as a nested list with visible indentation.

**How to reproduce it**:

Try and pass a DAG a `doc_md` string of the above nested list.
I think the bug will affect any markdown that relies on meaningful indentation (tabs or spaces)

airflow/www/utils.py
"""Mixin for filter for UTC time."""
    def apply(self, query, value):
        """Apply the filter."""
        value = timezone.parse(value, timezone=timezone.utc)
        return super().apply(query, value)  
class UtcAwareFilterEqual(UtcAwareFilterMixin, fab_sqlafilters.FilterEqual):  
    """Equality filter for UTC time."""
class UtcAwareFilterGreater(UtcAwareFilterMixin, fab_sqlafilters.FilterGreater):  
    """Greater Than filter for UTC time."""
class UtcAwareFilterSmaller(UtcAwareFilterMixin, fab_sqlafilters.FilterSmaller):  
    """Smaller Than filter for UTC time."""
class UtcAwareFilterNotEqual(UtcAwareFilterMixin, fab_sqlafilters.FilterNotEqual):  
    """Not Equal To filter for UTC time."""
class UtcAwareFilterConverter(fab_sqlafilters.SQLAFilterConverter):  
    """Retrieve conversion tables for UTC-Aware filters."""
    conversion_table = (
        (
            'is_utcdatetime',
            [UtcAwareFilterEqual, UtcAwareFilterGreater, UtcAwareFilterSmaller, UtcAwareFilterNotEqual],
        ),
    ) + fab_sqlafilters.SQLAFilterConverter.conversion_table
class CustomSQLAInterface(SQLAInterface):