Add "Tax Statements" section to handbook

Why is this change being made?

Earlier today I spent 10 minutes roaming around ADP's web portal looking for the W-2 they told me was available. Then I spent more time roaming around the handbook looking for tips. After a dozen searches I finally found the tip that helped me locate the W-2 in ADP. This was a dysfunctional process that we should help hundreds of others avoid.

The real problem is that ADP buries their "Tax Statements" section at the bottom of the "Pay Statements" page. That's not something we can fix.

What we can improve is the handbook. There was a great tip in the handbook, but I struggled to find it. It was buried in the "Pay Date" section. (It was also a bit outdated.)

The intent of this MR is to make "tax statement" information more discoverable in the handbook, and create a place where we can provide tax statement guidance for GitLab team members outside the US (PEO employees, contractors, etc.)

Author Checklist

• Provided a concise title for the MR
• Added a description to this MR explaining the reasons for the proposed change, per say-why-not-just-what
• Assign this change to the correct DRI
  • If the DRI for the page/s being updated isn’t immediately clear, then assign it to one of the people listed in the "Maintained by" section in on the page being edited.
  • If your manager does not have merge rights, please ask someone to merge it AFTER it has been approved by your manager in #mr-buddies.
  • If the changes relate to any part of the project other than updates to content and/or data files please make sure to ping @gl-static-site-editor in a comment for a review and merge. For example changes to .gitlab-ci.yml, JavaScript/CSS/Ruby code or the layout files. (this requirement has been removed pending identification of a new DRI for the handbook)