New Feature: "Why Ember" Page

This issue is to track work that is related to creating a "Why Ember" page for the new Ember Website. This is separate from the work that is happening in the [Ember Website Redesign RFC](https://github.com/emberjs/rfcs/pull/425)

### Motivation
It can sometimes be difficult to communicate why someone might want to consider using Ember as a technology, especially because there are many different kinds of stakeholders that would have different reasons to consider Ember. For example, many of the people in the Ember Community Discord are Ember developers that use Ember on a daily basis and could probably speak at length about some of the technical merits and potentially ergonomic benefits of using Ember, but might not have much of a grasp on the **business reasons** for using something like Ember. If you ask a develper they may talk about the powerful template syntax but a C-Level executivive might want to hear about our dedication to backwards compatibility and LTS release structure.

**Note:** `C-Level executive` refers to the decision makers of a business such as CTO, CEO or CIO. In the UK and other parts of the world they may be referred to as Managing Director or Technical Director. See https://en.wikipedia.org/wiki/Corporate_title for more information

While this effort doesn't proclude us from adding some sections in the theme of "Why Ember" to the homepage, having a whole page dedicated to it will allow us to give it the space it deserves.

### Research and Previous efforts
This initiative has been spearheaded by @MelSumner in early 2018 and there has been a lot of discussion and research put into collecting content and opinions for this section. There was a survery form circulated to the community in 2018 that was used to collect a wider sample of "Why people have chosen Ember".

@MelSumner has also published a [Gist here](https://gist.github.com/MelSumner/971ba6b7a3c0b01a4cb3a43d3b962dac) that was intended for people to have extra points of discussion that they might bring up when asked "Why should I use Ember"

### Requirements

- [ ] Add a route `/why` that will be used for this efforts. This means that https://emberjs.com/why will always link (or redirect) to something that helps to answer the question "Why should you consider using Ember"
- [ ] Add a section for Developers and group some of the content on why developers should consider using Ember
- [ ] Add a section for C-Level exeutives
- [ ] Collect responses from the "Why Ember" survey and decide with the Learning Team which items will be included in which sections

### Open Questions
- [ ] Do we need any other sections for the initial version? Should we have a section for project-managers or other parties?
- [ ] Would it be useful to have testimonials on this page?