feat: Add reassign contact doc and update sidebar

@andrewpaliga Here's a document for Reassigning Contacts. I felt it would be better as a stand alone from the Adding Contacts doc which was already handling 3 tasks, and this helps us avoid one more instance of the one-doc per section no-no.

I didn't have full knowledge on how a user can become a team member or receive admin privileges other than emailing hello@knokd.com - so there may be an opportunity for you to make this more accurate / helpful.

I think it's best if your team can review and give a 👍 up before 🚢 on this one. I'll get started on the Contact us section (global footer?) soon as well. Still got too many friends and family to see before I head back on the 28th but would love to get this all done for you asap too.

Knokd / docs.knokd.com

feat: Add reassign contact doc and update sidebar #24