Update wording, links in key projects page (#691)

* Update wording, links in key projects page

* Update and improve existing project descriptions

* Add new non-PyPA projects

* Add specifications reference for internal linking

* Improve working in source/key_projects.rst

Co-Authored-By: Brian Rutledge <brian@bhrutledge.com>

* Improve wording in source/key_projects.rst

Co-Authored-By: Brian Rutledge <brian@bhrutledge.com>

* Fix formatting in source/key_projects.rst

Co-Authored-By: Brian Rutledge <brian@bhrutledge.com>

* Improve wording in source/key_projects.rst

Co-Authored-By: Brian Rutledge <brian@bhrutledge.com>

* Improve wording in source/key_projects.rst

Co-Authored-By: Brian Rutledge <brian@bhrutledge.com>

* Improve formatting in source/key_projects.rst

Co-Authored-By: Sviatoslav Sydorenko <wk.cvs.github@sydorenko.org.ua>

* Add caution about Piwheels and PEP 503

Thanks to @webknjaz for the suggestion.

* Add note about location of Pipenv docs

Thanks to @bhrutledge for the suggestion.

* Add and improve project descriptions

* Improve formatting in source/key_projects.rst

Co-Authored-By: Sviatoslav Sydorenko <wk.cvs.github@sydorenko.org.ua>

* Improve virtualenv description in Key Projects

Thanks to pradyunsg for suggestion.

* Reduce inaccuracy regarding piwheels

Thanks to webknjaz for suggestion.

* Fix headers and formatting consistency for key projects

* Add webchat IRC links for key projects

Co-authored-by: Brian Rutledge <brian@bhrutledge.com>
Co-authored-by: Sviatoslav Sydorenko <wk.cvs.github@sydorenko.org.ua>