docs: label new features introduced in v0.4 (#1138)

In #1058, I noted:

> Tangentially related work:
> ...
> - better annotate new features (motivated by writing these docs)
>     - we should add "New" to new features :) 
>     - we should note when a feature was introduced (I think this is a core part of most software documentation)
>     - we should annotate things that are "Advanced" in so far as the average Just the Docs user will not use them / they require significant Jekyll knowledge
> 

This came up again in https://github.com/just-the-docs/just-the-docs/discussions/1136#discussioncomment-4716253, so I think it's best for us to resolve this sooner rather than later.

This PR is me doing that. I:

- have added a headings-level "New" label to every new heading introduced since `v0.3`
- added, when possible, inline YAML comments when new configuration options have been introduced

I did this by scanning through the CHANGELOG and selecting each feature that is either tagged with `Add` and has documentation.

I may have also missed any new features, so some double-checking would be helpful!

docs/ui-components/callouts.md

@@ -6,6 +6,10 @@ nav_order: 7
 ---
 
 # Callouts
+{: .d-inline-block }
+
+New (v0.4.0)
+{: .label .label-green }
 
 Markdown does not include support for callouts. However, you can style text as a callout using a Markdown extension supported by kramdown: [*block IALs*](https://kramdown.gettalong.org/quickref.html#block-attributes).
 

docs/ui-components/code.md

@@ -90,6 +90,10 @@ To demonstrate front end code, sometimes it's useful to show a rendered example
 ---
 
 ## Mermaid diagram code blocks
+{: .d-inline-block }
+
+New (v0.4.0)
+{: .label .label-green }
 
 [Mermaid](https://mermaid-js.github.io/mermaid/) allows you to add diagrams and visualizations using Markdown code blocks. **It is disabled by default**. However, you can turn on support for mermaid by adding a `mermaid` key to your `_config.yml`.
 
@@ -145,6 +149,10 @@ graph TD;
 *Note: for demonstration purposes, we've enabled mermaid on this site. It is still disabled by default, and users need to opt-in to use it.*
 
 ## Copy button
+{: .d-inline-block }
+
+New (v0.4.0)
+{: .label .label-green }
 
 The copy button for code blocks can be enabled or disabled via the `enable_copy_code_button` key in `_config.yml`. By default, the value of this key is `false`; users need to opt-in.
 
@@ -153,4 +161,4 @@ The copy button for code blocks can be enabled or disabled via the `enable_copy_
 enable_copy_code_button: true
 ```
 
-Note that this feature requires JavaScript; if JavaScript is disabled in the browser, this feature will not work.
+Note that this feature requires JavaScript; if JavaScript is disabled in the browser, this feature will not work.