Rename `Internals` section to `Concepts` to match core docs structure
and expand on how policies are evaluated.
Also include missing documentation for check grouping and fix examples
to use the new feature.
Move the Autoscaler agent configuration `source` to the `policy` page
since they are very closely related.
Also update all headers in this section so they follow the proper `h1 >
h2 > h3 > ...` hierarchy.
