Add examples to the dig man page

Add examples to the dig man page

The goal here is to help new or infrequent users figure out the most
basic ways to use dig.

Notes on the choice of examples:

* I wrote examples that users can copy and paste exactly as is, without
  having to come up with an appropriate IP address or domain name to use.
  The one exception is the `dig -x` example which uses an IP from the
  example range.
* `dig +noall +answer` here is because learning about `+noall +answer`
  was lifechanging for me when I learned about it, I've heard from
  others that they find it helpful too, and it's pretty hard to infer
  from the man page as is that it might be useful
* I thought about adding `+trace` but left it out because 5 examples was
  already starting to feel like a lot.