Add docs site issue board link

This MR adds the Category:Docs Site issue board that @sarahgerman set up to the monthly TW planning issue.

Pro:

This raises visibility to the team of docs site changes that are in the works. The docs site issue board work is Kanban style and not milestone specific, however it gives a great view of what work is at what stage at a time when folks are doing their monthly look ahead.

Con:

It's a busy issue already.

I think the value of visibility is worth the additional information.