Clarify the way forward

Following some discussions on Discord it is still not entirely clear what the way forward should be. I'm attempting to write down the current situation here and would welcome comment on how this is supposed to be changed:

- if `@ember/jquery` is **not used**
- if `jquery-integration` is **not set**
- the build shows a warning about either setting the flag or using the addon, otherwise same situation as `jquery-integration: enabled`
- if `jquery-integration is **enabled**
- `Ember.$` shows a deprecation for `Ember.$`
- `import $ from 'jquery';` shows a deprecation for `Ember.$`
- `this.$()` shows a deprecation for `this.$()`
- if `jquery-integration is **disabled**
- `Ember.$` throws an error
- `import $ from 'jquery';` throws an error
- `this.$()` throws an error
- if `@ember/jquery` is **used**
- if `jquery-integration is **enabled**
- `Ember.$` ??? (not tested)
- `import $ from 'jquery';` works without deprecation warning
- `this.$()` shows a deprecation for `this.$()`
- if `jquery-integration is **disabled**
- `Ember.$` throws an error
- `import $ from 'jquery';` works without deprecation warning
- `this.$()` throws an error

/cc @rwjblue @chancancode @simonihmig