Clarify version docs (#955)

This PR updates the home page and the CHANGELOG to refer to  v0.4.0.rc1 as a pre-release or release candidate, rather than a release. See [this comment](https://github.com/just-the-docs/just-the-docs/pull/613#issuecomment-1240442518) for motivation.

It also adds the versioned docs issue (#728) to the roadmap in the CHANGELOG.

As the config for the theme docs now needs to declare callouts, the [callouts docs](https://just-the-docs.github.io/just-the-docs/docs/ui-components/callouts/) can now illustrate the rendered appearance. (These callouts are merely examples: the names and colors should eventually be replaced by a principled collection, taking account of WCAG.)

_config.yml

@@ -107,6 +107,23 @@ gh_edit_view_mode: "tree" # "tree" or "edit" if you want the user to jump into t
 # Color scheme currently only supports "dark", "light"/nil (default), or a custom scheme that you define
 color_scheme: nil
 
+callouts_level: quiet # or loud
+callouts:
+  highlight:
+    color: yellow
+  important:
+    title: Important
+    color: blue
+  new:
+    title: New
+    color: green
+  note:
+    title: Note
+    color: purple
+  warning:
+    title: Warning
+    color: red
+
 # Google Analytics Tracking (optional)
 # e.g, UA-1234567-89
 ga_tracking: UA-2709176-10