Categorize NewestSha example under Db interface.

It seems the godoc example extraction does not recognize the same syntax
used for functions with receivers when working with interfaces.

So, instead, categorize the example on the interface.
Use magic syntax for godoc NewestSha example.
Example link godoc web don't match local godoc.
IMPORT PATH MODIFICATION CHANGESET
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Please use this changeset as the first one on top of the conformal codebase.
Note that if there are new imports, they need to be changed as part of this
changeset. In general, use "git rebase" on github.com/conformal's version
and apply this changeset first.