Render documentation for seed files

[clrcrl]

Seed files do not show in the side panel.
You can get to their docs by right clicking on the node in the DAG, and selecting "View Documentation". However, if you have documentation/tests for the seed files, it won't be rendered here.
Related to #15

[shedd]

It would be nice if there was a way to enhance the documentation with descriptions through a schema.yml file in the seed directory, as well. While I was able to find the documentation for my seed file as described above, DBT did not parse in the associated description from the schema.yml file that I added to the seed directory. The description on the page says This seed is not currently documented