docs: rename plugins index file and add linkTitle

We publish this page on docs.docker.com, and hugo expects index pages
for sections to be named _index.md. We currently rename the page when we
mount it to the docs repo but might as well change the filename in the
source.

Also adds a linkTitle to the page, which is a shorter title that will be
used in the sidebar navigation and breadcrumbs.

Signed-off-by: David Karlsson <35727626+dvdksn@users.noreply.github.com>

docs/extend/_index.md

@@ -1,5 +1,6 @@
 ---
 title: Docker Engine managed plugin system
+linkTitle: Docker Engine plugins
 description: Develop and use a plugin with the managed plugin system
 keywords: "API, Usage, plugins, documentation, developer"
 aliases:

docs/extend/legacy_plugins.md

@@ -8,7 +8,7 @@ keywords: "Examples, Usage, plugins, docker, documentation, user guide"
 
 This document describes the Docker Engine plugins generally available in Docker
 Engine. To view information on plugins managed by Docker,
-refer to [Docker Engine plugin system](index.md).
+refer to [Docker Engine plugin system](_index.md).
 
 You can extend the capabilities of the Docker Engine by loading third-party
 plugins. This page explains the types of plugins and provides links to several

docs/extend/plugin_api.md

@@ -8,7 +8,7 @@ Docker plugins are out-of-process extensions which add capabilities to the
 Docker Engine.
 
 This document describes the Docker Engine plugin API. To view information on
-plugins managed by Docker Engine, refer to [Docker Engine plugin system](index.md).
+plugins managed by Docker Engine, refer to [Docker Engine plugin system](_index.md).
 
 This page is intended for people who want to develop their own Docker plugin.
 If you just want to learn about or use Docker plugins, look

docs/extend/plugins_authorization.md

@@ -8,7 +8,7 @@ aliases:
 
 This document describes the Docker Engine plugins available in Docker
 Engine. To view information on plugins managed by Docker Engine,
-refer to [Docker Engine plugin system](index.md).
+refer to [Docker Engine plugin system](_index.md).
 
 Docker's out-of-the-box authorization model is all or nothing. Any user with
 permission to access the Docker daemon can run any Docker client command. The

docs/extend/plugins_network.md

@@ -6,7 +6,7 @@ keywords: "Examples, Usage, plugins, docker, documentation, user guide"
 
 This document describes Docker Engine network driver plugins generally
 available in Docker Engine. To view information on plugins
-managed by Docker Engine, refer to [Docker Engine plugin system](index.md).
+managed by Docker Engine, refer to [Docker Engine plugin system](_index.md).
 
 Docker Engine network plugins enable Engine deployments to be extended to
 support a wide range of networking technologies, such as VXLAN, IPVLAN, MACVLAN
@@ -19,7 +19,7 @@ kind of protocol.
 ## Network plugins and Swarm mode
 
 [Legacy plugins](legacy_plugins.md) do not work in Swarm mode. However,
-plugins written using the [v2 plugin system](index.md) do work in Swarm mode, as
+plugins written using the [v2 plugin system](_index.md) do work in Swarm mode, as
 long as they are installed on each Swarm worker node.
 
 ## Use network driver plugins