**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
"""Returns a formatted string with HTML for given JSON serializable"""
    def json_(attr):
        f = attr.get(attr_name)
        serialized = json.dumps(f)
        return Markup('<nobr>{}</nobr>').format(serialized)  
    return json_
def dag_link(attr):
    """Generates a URL to the Graph view for a Dag."""
    dag_id = attr.get('dag_id')
    execution_date = attr.get('execution_date')
    url = url_for('Airflow.graph', dag_id=dag_id, execution_date=execution_date)
    return Markup('<a href="{}">{}</a>').format(url, dag_id) if dag_id else Markup('None')  
def dag_run_link(attr):
    """Generates a URL to the Graph view for a DagRun."""
    dag_id = attr.get('dag_id')
    run_id = attr.get('run_id')
    execution_date = attr.get('execution_date')
    url = url_for('Airflow.graph', dag_id=dag_id, run_id=run_id, execution_date=execution_date)
    return Markup('<a href="{url}">{run_id}</a>').format(url=url, run_id=run_id)  
def pygment_html_render(s, lexer=lexers.TextLexer):