**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
"""Return URL-encoded params"""
    return urlencode({d: v for d, v in kwargs.items() if v is not None}, True)
def generate_pages(current_page, num_of_pages, search=None, status=None, tags=None, window=7):
    """
    Generates the HTML for a paging component using a similar logic to the paging
    auto-generated by Flask managed views. The paging component defines a number of
    pages visible in the pager (window) and once the user goes to a page beyond the
    largest visible, it would scroll to the right the page numbers and keeps the
    current one in the middle of the pager component. When in the last pages,
    the pages won't scroll and just keep moving until the last page. Pager also contains
    <first, previous, ..., next, last> pages.
    This component takes into account custom parameters such as search, status, and tags
    which could be added to the pages link in order to maintain the state between
    client and server. It also allows to make a bookmark on a specific paging state.
    :param current_page: the current page number, 0-indexed
    :param num_of_pages: the total number of pages
    :param search: the search query string, if any
    :param status: 'all', 'active', or 'paused'
    :param tags: array of strings of the current filtered tags
    :param window: the number of pages to be shown in the paging component (7 default)
    :return: the HTML string of the paging component
    """
    void_link = 'javascript:void(0)'
    first_node = Markup(
        """<li class="paginate_button {disabled}" id="dags_first">
    <a href="{href_link}" aria-controls="dags" data-dt-idx="0" tabindex="0">&laquo;</a>