**Apache Airflow version**: 2.0.0 - 2.1.0

**Kubernetes version**: N/A

**Environment**:

- **Cloud provider or hardware configuration**: Docker on MacOS (but also AWS ECS deployed)
- **OS** (e.g. from /etc/os-release): MacOS Big Sur 11.3.1
- **Kernel** (e.g. `uname -a`): Darwin Kernel Version 20.4.0
- **Install tools**:
- **Others**:

**What happened**:

When a code block is a part of a DAG's `doc_md`, it does not render correctly in the Web UI, but collapses all the lines into one line instead.

**What you expected to happen**:

The multi line code block be rendered with line breaks preserved.

**How to reproduce it**:
Create a DAG with `doc_md` containing a code block:

````python
from airflow import DAG

DOC_MD = """\
# Markdown code block

Inline `code` works well.

```
Code block
does not
respect
newlines
```
"""

dag = DAG(
    dag_id='test',
    doc_md=DOC_MD
)
````

The rendered documentation looks like this:
<img src="https://user-images.githubusercontent.com/11132999/119981579-19a70600-bfbe-11eb-8036-7d981ae1f232.png" width="50%"/>

**Anything else we need to know**: N/A


airflow/www/utils.py
"""Generates a URL to the Graph view for a TaskInstance."""
    dag_id = attr.get('dag_id')
    task_id = attr.get('task_id')
    execution_date = attr.get('execution_date')
    url = url_for('Airflow.task', dag_id=dag_id, task_id=task_id, execution_date=execution_date.isoformat())
    url_root = url_for(
        'Airflow.graph', dag_id=dag_id, root=task_id, execution_date=execution_date.isoformat()
    )
    return Markup(  
        """
        <span style="white-space: nowrap;">
        <a href="{url}">{task_id}</a>
        <a href="{url_root}" title="Filter on this task and upstream">
        <span class="material-icons" style="margin-left:0;"
            aria-hidden="true">filter_alt</span>
        </a>
        </span>
        """
    ).format(url=url, task_id=task_id, url_root=url_root)
def state_token(state):
    """Returns a formatted string with HTML for a given State"""
    color = State.color(state)
    fg_color = State.color_fg(state)
    return Markup(  
        """
        <span class="label" style="color:{fg_color}; background-color:{color};"
            title="Current State: {state}">{state}</span>
        """
    ).format(color=color, state=state, fg_color=fg_color)
def state_f(attr):