**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
import warnings
    from airflow.utils.log.secrets_masker import get_sensitive_variables_fields
    warnings.warn(
        "This function is deprecated. Please use "
        "`airflow.utils.log.secrets_masker.get_sensitive_variables_fields`",
        DeprecationWarning,
        stacklevel=2,
    )
    return get_sensitive_variables_fields()
def should_hide_value_for_key(key_name):  
    import warnings
    from airflow.utils.log.secrets_masker import should_hide_value_for_key
    warnings.warn(
        "This function is deprecated. Please use "
        "`airflow.utils.log.secrets_masker.should_hide_value_for_key`",
        DeprecationWarning,
        stacklevel=2,
    )
    return should_hide_value_for_key(key_name)
def get_params(**kwargs):