USWDS - About: add security updates.

[mejiaj]

Description

Closes #1297. Added a section for security related updates on USWDS. This new content is part of a collection called _security_updates. A security update contains:

---
title: What we did in present tense
excerpt: A brief description of when the change happened and what it does
---

Description

## When we made this change
Link to which version it was fixed in.

## Who should update
What versions of USWDS does this affect.

## Updating to USWDS 2.x.x
List of breaking changes when upgrading.

Additional information

N/A

Before you hit Submit, make sure you’ve done whichever of these applies to you:

• Follow the 18F Front End Coding Style Guide and Accessibility Guide.
• Run npm test and make sure the tests for the files you have changed have passed.
• Run your code through HTML_CodeSniffer and make sure it’s error free.
• Title your pull request using this format: [Website] - [UI component]: Brief statement describing what this pull request solves.

[mejiaj]

This ties the content in security_updates collection to Security in sidenav. Otherwise the current status doesn't display properly.

URLs don't actually work

[crazEmama1]

L