Provide page titles, align titles, pluralize

This makes us more consistent about having a Registries (plural)
page, with a matching title, with each registry being more consistent
about its linking and name.

This does not make everything perfectly consistent due to lack of
time to track down all of the magical places where Jekyll or Liquid
does whatever it does.  For example, I cannot find where "Tag Kind"
singular comes from in one list when it is plural everywhere else,
among other minor inconsistencies.

Author: handrews

registry/index.md

@@ -1,17 +1,21 @@
 ---
-title: Registry
+title: Registries
 layout: default
 permalink: /registry/index.html
 has_children: true
 children:
-- title: Alternative Schema Registry
+- title: Alternative Schema Type Registry
 - title: Draft Features Registry
+- title: Specification Extension Registry
 - title: Format Registry
+- title: Media Type Registry
 - title: Namespace Registry
 - title: Tag Kinds Registry
 has_toc: false
 ---
 
+# Registries
+
 ## Contributing
 
 Please raise a [Pull-Request](https://github.com/OAI/OpenAPI-Specification/pulls) against the `gh-pages` branch and add a new Markdown file to a folder in the `registries/_{registryName}`. The name of the file is considered the registration entry, ignoring the file extension. Alternatively you can open an [Issue](https://github.com/OAI/OpenAPI-Specification/issues) to discuss a registry value.