foo/README.md and bar/README.md don't clobber each other.

'./pants goal markdown ::' has a problem: every .../README.md file generates to the same path (.pants.d/markdown/markdown/html/README.html)

With this change, you instead get
.pants.d/markdown/markdown/html/_.readme.html
.pants.d/markdown/markdown/html/_src.java.com.pants.examples.hello.main.readme.html
.pants.d/markdown/markdown/html/_src.java.com.pants.testproject.page.readme.html
.pants.d/markdown/markdown/html/_src.python.pants.backend.python.readme.html
.pants.d/markdown/markdown/html/_src.python.pants.readme.html

Also in this change:
Turned some unnecessarily complex python into simple mustache templates.

Added src/java/com/pants/testproject/page to make sure I didn't break [[pants(...)]] "wikilinks".
Doesn't contain java, despite the dir.

Testing Done:
(got confluence goal temporarily working to make sure that markdown-page-moves didn't break confluence publish)

https://travis-ci.org/pantsbuild/pants/builds/31464526

Reviewed at https://rbcommons.com/s/twitter/r/750/