cmd/inspect: render "related resources" and small misc changes (#4399)

Some small changes:

* cmd/inspect: inline ast.DropHead() as dropDataPrefix() -- to avoid the extra export in the ast package
* docs/annotations: update related_resources example
* cmd/inspect: unexport PageWidth
* cmd/inspect: make usage text markdown-friendly

   This looks better on the website docs, and doesn't matter much when
   viewed on the CLI.

* cmd/inspect: add test for pretty annotations output
* cmd/inspect: also render related_resources in pretty mode

Signed-off-by: Stephan Renatus <stephan.renatus@gmail.com>