* docs: mention PYTHONPATH in installing.rst

When pybind11 is included as a submodule, the user needs to update their
Python module search path.  Otherwise, the first c++ compilation command
in docs/basics.rst will fail.

* docs: add a note about compiling the example

This note shows how to modify the compilation command for the example
when the pybind11 source has been included as a Git submodule.

* docs: add a note about compiling the example

Added an internal link to the docs

* docs: updated a note about compiling the example

Also updated the command substitution syntax for consistency