Fix broken links and some spelling

This MR:

1. Fixes most of the broken links leftover from gitlab-com/content-sites/handbook-tools/maintenance-tasks#3 (closed) and any new ones found as part of gitlab-com/content-sites/handbook-tools/maintenance-tasks#4 (closed)
2. Adds a bunch of spelling exceptions
3. Fixes a number of spelling mistakes encountered
4. Resolves #308 (closed)