Update A_train.md

[RaneyDazed]

I added some of the autoscan docs to the a-train page. added it to mkdocs.yml. I wasn't sure the direction you guys wanted to go with this, so I just set this up and figured you could dick with it if it wasn't to your liking.

• [x] App documentation page created
• [x] Reference added in docs/sandbox/index.md
• [x] Reference added in mkdocs.yml