to the understanding links, add the level

[shawna-slh]

e.g., under Learn More

current:

Understanding 2.4.2: Page Titled

revised:

Understanding 2.4.2: Page Titled (Level A)

Rationale: This provides users more info without adding significant clutter. (It also better matches the Understanding page's h1.)

Also, we got this e-mail: "Is it possible for you to let me know if the recommendations in the easy checks are either levels A, AA, or AAA?"