Reorganize docs contrib guide (#9510)

* Reorganize docs contrib guide

* Address first round of feedback from Brad, Jared

* Standardize on 'SIG Docs'

* Address more feedback

* Rewrites to participating.md

* Tweak navigation titles

* Document PR Wrangler

* Document SIG Docs chairperson

* Fix codeblock that shows how to use <codenew>

It was being interpreted as a Hugo shortcode.

content/en/docs/contribute/generate-ref-docs/_index.md

@@ -0,0 +1,9 @@
+---
+title: Reference docs overview
+main_menu: true
+weight: 80
+---
+
+Much of the Kubernetes reference documentation is generated from Kubernetes
+source code, using scripts. The topics in this section document how to generate
+this type of content.