doc: dust off the Task Developer's Guide :-) rm the "pagerank" example

Our page about how to develop a Task described "science-era" Pants.

Removed a lot of what's there; it talked about APIs that have since become new and improved.

Removed the "pagerank" example. :-( When I started on this change, I thought I was gonna update this example and maybe move it to examples/src/... so it would get exercised and not drift back into b0rken-ness . But it's kind of a weird example--it doesn't really care about products, it doesn't do "the usual" things with deps. So I find myself deleting it... but with some misgivings, because it's cool. If someone wants to do the work to modernize it and move it to examples, I'll happily code review :-)

Replaced copy-pasted code snippets with "includes" from live tasks.
Maybe this won't drift out of date so easily from now on.
(Didn't add includes for the JVM section, just a TODO. But did remove now-obsolete sample code.)

rm'd some redundant-and-obsolete info from the Internals page, replacing w/link to Task Developer's guide. (Thanks to Mateo for spotting this!) The basic concepts were OK, but the details, yikes. Internals page had info about GroupTask, which Task Developers page lacked... so move it over.

Bugs closed: 433

Reviewed at https://rbcommons.com/s/twitter/r/1074/
Sync of https://github.com/pantsbuild/pants/commit/63c0a02f3f4a26be40e69e42a262cc99720d4257