Adding direct link to PMM team from Handbook TOC as it was omitted from there....

Why is this change being made?

As I was reading the Handbook, I noticed there was no mention of Product Marketing in the main TOC. After browsing to Strategic Marketing, I realized the PMM team is in fact a part of Strategic Marketing along with a few others; still, in the main TOC, it wasn't mentioned. I thought this was confusing, so I added the direct link to the PMM team from Handbook TOC. The order of teams is the same as under Strategic Marketing (https://about.gitlab.com/handbook/marketing/product-marketing/)