DOC: update bento docs (close #2993).
BUG: numpydoc/linkcode: do not detect linkcode config changes

Since the linkcode_resolve is a function, it triggers configuration
change on each rebuild, resulting to full rebuild always. It's better
to make it not do that.
DOC: numpydoc/linkcode: mention that the extension will be in Sphinx upstream
ENH: numpydoc: deal with duplicated signatures
ENH: numpydoc: Python 2 & 3 in single codebase, restructure as a package
MAINT: always use plot directive from Matplotlib, and prefer Sphinx linkcode
TST: numpydoc: add stub test files, to check that files at least import
BUG: numpydoc: fix bugs in attribute docstring extraction + improve presentation
TST: numpydoc: more class tests
BUG: numpydoc: check that it works with sub-classes