**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
rendered = wrapped_markdown("*italic*\n**bold**\n", css_class="a_class")
        assert (
            '''<div class="a_class" ><p><em>italic</em>
<strong>bold</strong></p></div>'''
            == rendered
        )
    def test_wrapped_markdown_with_table(self):
        rendered = wrapped_markdown(
            """| Job | Duration |
               | ----------- | ----------- |
               | ETL | 14m |"""
        )
        assert (
            '<div class="rich_doc" ><table>\n<thead>\n<tr>\n<th>Job</th>\n'
            '<th>Duration</th>\n</tr>\n</thead>\n<tbody>\n<tr>\n<td>ETL'
            '</td>\n<td>14m</td>\n</tr>\n</tbody>\n'
            '</table></div>'
        ) == rendered
    def test_wrapped_markdown_with_indented_lines(self):
        rendered = wrapped_markdown(
            """
                # header
                1st line
                2nd line
            """
        )
        assert '<div class="rich_doc" ><h1>header</h1>\n<p>1st line\n2nd line</p></div>' == rendered