docs: branch is now a variable

This makes our docs less brittle and fixes a broken link for pull
requests against known plugins.

docs/development/plugin/development-plugin-resources.asciidoc

@@ -24,5 +24,5 @@ Check out the https://github.com/elastic/generator-kibana-plugin[plugin generato
 
 [float]
 ==== References in the code
- - {repo}/blob/master/src/server/plugins/plugin.js[Plugin class]: What options does the `kibana.Plugin` class accept?
+ - {repo}blob/{branch}/src/server/plugins/plugin.js[Plugin class]: What options does the `kibana.Plugin` class accept?
  - <<development-uiexports>>: What type of exports are available?

docs/index.asciidoc

@@ -7,9 +7,10 @@ release-state can be: released | prerelease | unreleased
 :release-state:  unreleased
 :version:        6.0.0-alpha1
 :major-version:  6.x
+:branch:         master
 
 :docker-image:   docker.elastic.co/kibana/kibana:{version}
-:es-ref:         https://www.elastic.co/guide/en/elasticsearch/reference/master/
+:es-ref:         https://www.elastic.co/guide/en/elasticsearch/reference/{branch}/
 :xpack-ref:      https://www.elastic.co/guide/en/x-pack/current/
 :repo:           https://github.com/elastic/kibana/
 :issue:          {repo}issues/

docs/plugins/known-plugins.asciidoc

@@ -38,4 +38,4 @@ This list of plugins is not guaranteed to work on your version of Kibana. Instea
 === Other
 * https://github.com/nreese/kibana-time-plugin[Time picker as a dashboard panel] Widget to view and edit the time range from within dashboards.
 
-NOTE: If you want your plugin to be added to this page, open a {repo}tree/{version}/docs/plugins/known-plugins.asciidoc[pull request].
+NOTE: If you want your plugin to be added to this page, open a {repo}tree/{branch}/docs/plugins/known-plugins.asciidoc[pull request].