Move 'Plugins Source Code Hosting' documentation to an own page

This part got a bit lost in the huge dev-plugins.txt which describes all
the extension points. Add a separate page for this and link to it more
prominently so that it can be discovered more easily

Also rename this documentation to "Plugin Lifecycle" as this title seems
to be more accurate.

Bug: Issue 11638
Signed-off-by: Edwin Kempin <ekempin@google.com>
Change-Id: I44b1539918701232db5d4f4d9372e09b17175248

Documentation/dev-community.txt

diff --git a/Documentation/dev-community.txt b/Documentation/dev-community.txt
index 0656090..591451b 100644
--- a/Documentation/dev-community.txt
+++ b/Documentation/dev-community.txt
@@ -49,6 +49,7 @@
 
 [[plugin-development]]
 == Plugin Development
+* link:dev-plugins-lifecycle.html[Plugin Lifecycle]
 * link:dev-plugins.html[Developing Plugins]
 * link:dev-build-plugins.html[Building Gerrit plugins]
 * link:js-api.html[JavaScript Plugin API]