Public-Sans: Add Github action add-to-project

[amyleadem]

Description

Related to issue uswds/uswds-team#176

Purpose

Add the Github add-to-project action to our repos so that newly opened issues are automatically added to our project board.

⚠️ This PR uses a 7-day PAT (issued on 06/27) in the encrypted secret for testing purposes. We will re-evaluate the use of this action after that 7 day period is up. If we decide to keep this action, we will update the PAT to one with a longer life.

Demo and testing

This action works successfully on my personal test repo. To test on this repo:

1. First open a new issue
2. See that the new issue is automatically added to the sample project board

To test on this repo:

1. Merge this PR
2. Open a new issue
3. See if it lands on our project board