**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
disabled=is_disabled,
        )
    )
    page_link = void_link
    if current_page > 0:
        page_link = f'?{get_params(page=current_page - 1, search=search, status=status, tags=tags)}'
    output.append(previous_node.format(href_link=page_link, disabled=is_disabled))  
    mid = int(window / 2)
    last_page = num_of_pages - 1
    if current_page <= mid or num_of_pages < window:
        pages = list(range(0, min(num_of_pages, window)))
    elif mid < current_page < last_page - mid:
        pages = list(range(current_page - mid, current_page + mid + 1))
    else:
        pages = list(range(num_of_pages - window, last_page + 1))
    def is_current(current, page):  
        return page == current
    for page in pages:
        vals = {
            'is_active': 'active' if is_current(current_page, page) else '',
            'href_link': void_link
            if is_current(current_page, page)
            else f'?{get_params(page=page, search=search, status=status, tags=tags)}',
            'page_num': page + 1,
        }
        output.append(page_node.format(**vals))  
    is_disabled = 'disabled' if current_page >= num_of_pages - 1 else ''
    page_link = (
        void_link
        if current_page >= num_of_pages - 1