New sidebar navigation, updated titles (#7643)

1. Updates the folder structure of the main sidebar navigation to [a new
version](https://unleash-internal.slack.com/archives/C06NG1LUUCU/p1721413054821399)
2. Updates the title of `15 Principles for using feature flag systems at
scale` article to a new title given by @riboflavin
3. Updates the title of `11 Principles for building and scaling feature
flag systems` article to a new title given by @riboflavin


## Before (all collapsed)


![image](https://github.com/user-attachments/assets/1cd613e6-4752-4a58-9c28-136eb59d1336)


## After (all collapsed)


![image](https://github.com/user-attachments/assets/09546d9e-3049-4862-8139-2264541d436e)