Add an entry to the Setuptools usage documentation for the
project_urls dict, and include it in the "advanced" project example.

While at it, adjust the dogfooding use in Setuptools' own setup.py
to use HTTPS for the Documentation URL and drop redundant entries
for Bug Tracker and Source Code.

Also remove a no-op attribute reassignment from egg_info.py.

By including one or more Project-URL entries in PKG-INFO metadata,
PyPI can display helpful hyperlinks in a generic manner. Add support
here to be able to pass it through setup.cfg and setup.py with a
project_urls dict. See the corresponding section of the Core
Metadata Specifications from the Python Packaging User Guide for
details:

https://packaging.python.org/specifications/core-metadata/#project-url-multiple-use

Now it's possible to use it like this:
* long_description = file: README.rst, CHANGELOG.rst, LICENSE.rst