Add issue tracker to sales, move sales-process to "just" sales

@ChadMalchow, I made this MR in line with https://dev.gitlab.org/gitlab/organization/issues/846 where we are trying to align the handbook page structure with the company structure.

Let me know if you have any concerns here. As a next step, I would imagine bringing the subfolders that are now under sales-process over to be sub-folders of "just" sales. Let me know if you have other structures or wishes in mind and I'll be happy to help.