New section "Custom documentation"

The custom documentation links are still under the "Learn SurveyStack" section, but we really need that to be in a separate section. I believe we had a separate issue that described that fix, but definitely we should fix this ASAP.

In addition, newly added documentation doesn't appear until you hard refresh the page. Ideally, this should appear after adding... so I would consider refreshing the page after a user adds new documentation so it shows up (that's helpful for the user to see the change... otherwise they changed something but it didn't appear to work). See item below "When you save in group -> settings, there's no confirmation" for potentially related issue and solution.

https://miro.com/app/board/uXjVKJgAr3w=/?moveToWidget=3458764597106267970&cot=14