**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
</li>"""
    )
    previous_node = Markup(
        """<li class="paginate_button previous {disabled}" id="dags_previous">
    <a href="{href_link}" aria-controls="dags" data-dt-idx="0" tabindex="0">&lsaquo;</a>
</li>"""
    )
    next_node = Markup(
        """<li class="paginate_button next {disabled}" id="dags_next">
    <a href="{href_link}" aria-controls="dags" data-dt-idx="3" tabindex="0">&rsaquo;</a>
</li>"""
    )
    last_node = Markup(
        """<li class="paginate_button {disabled}" id="dags_last">
    <a href="{href_link}" aria-controls="dags" data-dt-idx="3" tabindex="0">&raquo;</a>
</li>"""
    )
    page_node = Markup(
        """<li class="paginate_button {is_active}">
    <a href="{href_link}" aria-controls="dags" data-dt-idx="2" tabindex="0">{page_num}</a>
</li>"""
    )
    output = [Markup('<ul class="pagination" style="margin-top:0;">')]
    is_disabled = 'disabled' if current_page <= 0 else ''
    first_node_link = (
        void_link if is_disabled else f'?{get_params(page=0, search=search, status=status, tags=tags)}'
    )
    output.append(
        first_node.format(
            href_link=first_node_link,