**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
"""Gets 'state' & returns a formatted string with HTML for a given State"""
    state = attr.get('state')
    return state_token(state)
def nobr_f(attr_name):
    """Returns a formatted string with HTML with a Non-breaking Text element"""
    def nobr(attr):
        f = attr.get(attr_name)
        return Markup("<nobr>{}</nobr>").format(f)  
    return nobr
def datetime_f(attr_name):
    """Returns a formatted string with HTML for given DataTime"""
    def dt(attr):  
        f = attr.get(attr_name)
        as_iso = f.isoformat() if f else ''
        if not as_iso:
            return Markup('')
        f = as_iso
        if timezone.utcnow().isoformat()[:4] == f[:4]:
            f = f[5:]
        
        return Markup('<nobr><time title="" datetime="{}">{}</time></nobr>').format(as_iso, f)  
    return dt
def json_f(attr_name):