Split a pyproject.toml guide out of the "Declaring project metadata" specification

Fixes pypa#1349

Author: jeanas

source/guides/creating-and-discovering-plugins.rst

@@ -116,6 +116,8 @@ a list of packages to :func:`setup`'s ``packages`` argument instead of using
     :doc:`packaging-namespace-packages` documentation and clearly document
     which approach is preferred for plugins to your project.
 
+.. _plugin-entry-points:
+
 Using package metadata
 ======================
 

source/guides/index.rst

@@ -18,6 +18,7 @@ introduction to packaging, see :doc:`/tutorials/index`.
    :maxdepth: 1
    :caption: Building and Publishing Projects:
 
+   writing-pyproject-toml
    distributing-packages-using-setuptools
    using-manifest-in
    single-sourcing-package-version