Document addon suits/monorepos with independent versions

This discussion emerged in slack and I will try to put everything together in this issue:

## Background

Open sources projects as well as companies internal projects are sometimes put together as a monorepo, i.e. to group all addons they maintain under one umbrella. These individual addons may be published either individually (= independent) or in one batch (= each addon is at the same version, an example for this is `ember-decorators`).
In order to use ember-cli-addon-docs with those setups isn't as simple as it should be. So I'll outline what are the workarounds today and then bring in some ideas for future solutions.

## Workaround 1: Multi-Documentation Multi-Version (monorepo)

In that case, each addon will have it's own ember-cli-addon-docs. Each docs will only document it's own addon, completely independent of each other.

**Pros**
- ✅ Documentation sticks to the addon

**Cons**
- ⚠️ Cannot be versioned as it is using the git tag (there is only one tag for the repo not for each addon inside)
- ⚠️ Cannot be put up as github page (as there is only one allowed per repo). Must be put in a subdiretory to mimic monorepo (= additional setup)

## Workaround 2: Mono-Documentation Mono-Version (monorepo)

In that scenario an umbrella addon exists (like for`ember-decorators`) that has all the other addons as dependencies and provides the documentation to them. All addons share the same version.

**Pros**
- ✅ Works out of the box
- ✅ Versioned docs behave adequate to the published versions
- ✅ Is good to document things like addon suites

**Cons**
- ⚠️ No distinction on the docs between different addons, they are represented as ONE (makes sense for projects like ember-decorators).

## Workaround 3: Mono-Documentation Multi-Version (monorepo)

The umbrella addon depends on all other addons. Documentation is built in one place.

**Pros**
- ✅ Documentation is in one place
- ✅ Is good to document things like addon suites
- ✅ Navigation for projects is up to the author

**Cons**
- ❗️ Documentation is in one place
- ❗️ Navigation has no convention yet, is up to the author
- ⚠️ Requires some broccoli work to "load" dependent addons as projects, see [sandbox example](https://github.com/ember-learn/ember-cli-addon-docs/blob/7a85f7f877557c2023c7243864741d0b38cc01b0/ember-cli-build.js#L11) (= additional setup)
- 🚫 CONFLICTS with versioning (git tag-wise)

## Workaround 4: Multi-Documentation Multi-Version (monorepo)

Each addon has it's own documentation and it's own versioning (indepdent of others).

**Pros**
- ✅ One documentation for One addon

**Cons**
- ☹️ Documentation of suites is cluttered and spread across different sites
- ⚠️ Cannot be put up as github page (as there is only one allowed per repo). Must be put in a subdiretory to mimic monorepo (= additional setup)
- 🚫 CONFLICTS with version (git tag-wise)

## Workaround 5: Multi-Documentation Multi-Version (multirepo)

Each addon is its own repo, has its own documentation and its own versioning. This looks like the most viable solution as of today.

**Pros**
- ✅ Works today
- ✅ Versions do work
- ✅ Works with github pages

**Cons**
- ☹️ Documentation of suites is cluttered

## Solution

Ok, let's talk about a paradise situation that I am imagining in the following lines (I might just be 1.60m short but tall enough to think big 😁):

1. I do see [emberobserver](https://www.emberobserver.com/) rewritten as engine and be put up on the ember website (see this [sketch](https://i.imgur.com/rUjsZ5J.png) and my [tweet](https://twitter.com/unistyler/status/1009054487284199425) 😉).
2. Instead of/alternative to putting up documentation on github pages, provide a hosting service for documentation, similarly to what esdoc is doing: https://doc.esdoc.org/
3. Integrate emberobserver with these documentations
4. win. win. win.
5. Time to celebrate 🌴 🏖

This would mean, there is one documentation for one addon. Drop the idea of documenting suites, although it is one thing a developer can be very proud of to show everything in one place which gives some sort of portfolio.

In the meantime, emberobserver can scan `package.json` for the dependency of `ember-cli-addon-docs`, the keyword `ember-addon` and `homepage` field, create an index and provide seach for docs upon that (or mark documentation on each addon page). Something like that (I don't even know what EO is capable of as of today 🙈).