Start a separate documentation page for core plugins

Core plugins are described as part of the plugin config documentation.
The purpose of the plugin config documentation is to give an overview of
all plugins and which functionality they provide. Defining what core
plugins are doesn't really fit into this page. A separate documentation
page for core plugins makes sense since we intend to expand this
documentation. We want to define more clearly what a core plugin is,
what criteria a plugin must fulfill to become a core plugin and
processes around core plugins. All this newly planned documentation
around core plugins is surely out of scope for the plugin config
documentation, hence we want to have a new page for this.

Signed-off-by: Edwin Kempin <ekempin@google.com>
Change-Id: If6f1867cd4ec0a5843080e094cb397c30af99db8

Documentation/dev-core-plugins.txt

@@ -0,0 +1,28 @@
+= Gerrit Code Review - Core Plugins
+
+[[definition]]
+== What are core plugins?
+
+Core plugins are plugins that are packaged within the Gerrit war file. This
+means during the link:pgm-init.html[Gerrit initialization] they can be easily
+installed without downloading any additional files.
+
+To make working with core plugins easy, they are linked as
+link:https://gerrit.googlesource.com/gerrit/+/refs/heads/master/.gitmodules[Git
+submodules] in the `gerrit` repository. E.g. this means they can be easily
+link:dev-readme.html#clone[cloned] together with Gerrit.
+
+The core plugins are developed and maintained by the Gerrit maintainers
+and the Gerrit community.
+
+[[list]]
+== Which core plugins exist?
+
+See link:config-plugins.html#core-plugins[here].
+
+GERRIT
+------
+Part of link:index.html[Gerrit Code Review]
+
+SEARCHBOX
+---------