revise pytest docs with code-block & cross-refs

add pytest to intersphinx links in conf.py
refactoring comma etcetera
update pytest version dependency & recommendation
add (non-inline) hyperlink to pytest.org
one combined code-block:: bash for all command-line examples
eliminate Plugins section: importorskip() and deprecated_function() are builtin
rewrite skipping text to reflect above, drop hyperlink for :ref:/:func: below
add subsubsections for each function and decorator
add intersphinx :func: cross-reference for importorskip()
code-block:: python for all examples
use :ref: for skipif - expands to subsection title, so reworded to match
note decorators on classes require Python 2.6
use :ref: for xfail - expands to subsection title, so reworded to match
expand explanation of xfail decorator parameters, fix bug in simple example
intersphinx :func: for deprecated_call(); as it's builtin, now before recwarn
italicized hyperlink to recwarn as it lacks defined :ref: target