**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)

tests/www/test_utils.py
def check_generate_pages_html(self, current_page, total_pages, window=7, check_middle=False):
        extra_links = 4  
        search = "'>\"/><img src=x onerror=alert(1)>"
        html_str = utils.generate_pages(current_page, total_pages, search=search)
        assert search not in html_str, "The raw search string shouldn't appear in the output"
        assert 'search=%27%3E%22%2F%3E%3Cimg+src%3Dx+onerror%3Dalert%281%29%3E' in html_str
        assert callable(html_str.__html__), "Should return something that is HTML-escaping aware"
        dom = BeautifulSoup(html_str, 'html.parser')
        assert dom is not None
        ulist = dom.ul
        ulist_items = ulist.find_all('li')
        assert min(window, total_pages) + extra_links == len(ulist_items)
        page_items = ulist_items[2:-2]
        mid = int(len(page_items) / 2)
        for i, item in enumerate(page_items):
            a_node = item.a
            href_link = a_node['href']
            node_text = a_node.string
            if node_text == str(current_page + 1):
                if check_middle:
                    assert mid == i
                assert 'javascript:void(0)' == href_link
                assert 'active' in item['class']
            else:
                assert re.search(r'^\?', href_link), 'Link is page-relative'
                query = parse_qs(href_link[1:])
                assert query['page'] == [str(int(node_text) - 1)]
                assert query['search'] == [search]
    def test_generate_pager_current_start(self):