Add fieldable documentation entity

Some possible approaches

1. Configuration entity -
+ This has the pro of having the values wide with config so it exports an imports the the values can be changed along with any actual config changes.
- The drawback is that I have not found a way to have fieldable config entities.

2. Content entity of its own entity type so that it does not intermingle with node bundles.
+ The advantage is that it is easily fieldable.
- Drawback it that it is not natively exportable/importable.

3. Third party settings.
+ This is the proper use for third party settings so it makes sense to use it.
+ exports as config
- unknown if a View can access them
- to make it flexible/"fieldable" I think it might take a form alter (and hopefully not much more.

I moved the pathing and tab requirement to separate issues.

This has been released as part of alpha4.
There are still a couple of known issues.

1. Revisons are not working
2. Some bundles in the name of the document show machine name instead of human name.