**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
"""Mixin for filter for UTC time."""
    def apply(self, query, value):
        """Apply the filter."""
        value = timezone.parse(value, timezone=timezone.utc)
        return super().apply(query, value)  
class UtcAwareFilterEqual(UtcAwareFilterMixin, fab_sqlafilters.FilterEqual):  
    """Equality filter for UTC time."""
class UtcAwareFilterGreater(UtcAwareFilterMixin, fab_sqlafilters.FilterGreater):  
    """Greater Than filter for UTC time."""
class UtcAwareFilterSmaller(UtcAwareFilterMixin, fab_sqlafilters.FilterSmaller):  
    """Smaller Than filter for UTC time."""
class UtcAwareFilterNotEqual(UtcAwareFilterMixin, fab_sqlafilters.FilterNotEqual):  
    """Not Equal To filter for UTC time."""
class UtcAwareFilterConverter(fab_sqlafilters.SQLAFilterConverter):  
    """Retrieve conversion tables for UTC-Aware filters."""
    conversion_table = (
        (
            'is_utcdatetime',
            [UtcAwareFilterEqual, UtcAwareFilterGreater, UtcAwareFilterSmaller, UtcAwareFilterNotEqual],
        ),
    ) + fab_sqlafilters.SQLAFilterConverter.conversion_table
class CustomSQLAInterface(SQLAInterface):