More docs updates

We did a bit of cleanup around our documentation and added more information about what is happening at an architecture level. We already had the latter in our self-hosted documentation, but we have never documented or shared that this is the exact same architecture we use internally as well. So now you can get an idea of what is happening under the hood when you use Depot, when to use it, and when it makes sense to not use it. Check it out in our introduction docs.