Add generic specification for the Eclipse provisioning platform (p2)

In short, a p2 unit can be described via p2/<id>/<version>.

PURL-TYPES.rst

@@ -397,6 +397,59 @@ nuget
 
       pkg:nuget/[email protected]
 
+p2
+----
+``p2`` for Eclipse p2 units:
+
+- A p2 unit represents a unit of software that can be consumed using the Eclipse
+  Provisioning Platform. A unit can represent various types of software 
+  artifacts, such as Eclipse features, plug-ins or other bundles.
+
+- A p2 unit is uniquely identified by its id and version. A namespace is
+  therefore not necessary.
+
+- The name corresponds to the unique identifier of a given p2 unit. This id
+  uniquely identifies a unit within the Eclipse Provisioning Platform. 
+  Examples::
+
+      org.eclipse.core.resources.win32.x86_64
+      org.eclipse.rcp.sdk.id.executable.win32.win32.x86_64
+      org.eclipse.sdk.feature.group
+
+- The version corresponds to the version of the p2 unit. A p2 repository may
+  host multiple versions of the same unit. This information is therefore
+  essential when resolving the relationship between different artifacts.
+  Examples::
+
+      3.5.500.v20220812-1420
+      2.0.0.202304281106
+
+- The software artifact are accessed from a p2 repository. Given that each
+  project may host their own update site, no default repository exists. The base
+  path to this repository is provided by the ``repository_url`` qualifier key
+  and is optional.
+  Examples::
+
+       https://download.eclipse.org/releases/2023-09/202309131000
+       https://download.itemis.com/updates/releases/2.1.1
+
+- A p2 repository can host a multitude of artifacts. The type of artifact is
+  provided by the ``classifier`` qualifier key and is optional.
+  Examples::
+
+        binary
+        org.eclipse.update.feature
+        osgi.bundle
+
+
+- The full URL of a p2 unit can be derived from the name, version, repository
+  URL and classifier.
+  Examples::
+
+      pkg:p2/[email protected]
+      pkg:p2/org.eclipse.rcp.sdk.id.executable.win32.win32.x86_64@4.29.0.I20230903-1000?classifier=binary
+      pkg:p2/[email protected]?repository_url=https://download.eclipse.org/releases/2023-09/202309131000
+
 qpkg
 ----
 ``qpkg`` for QNX packages:
@@ -558,7 +611,6 @@ Other candidate types to define:
 - ``opam`` for OCaml packages:
 - ``openwrt`` for OpenWRT packages:
 - ``osgi`` for OSGi bundle packages:
-- ``p2`` for Eclipse p2 packages:
 - ``pear`` for Pear PHP packages:
 - ``pecl`` for PECL PHP packages:
 - ``perl6`` for Perl 6 module packages: