**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
</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,