You're right that the Markua spec (http://markua.com/) is very long, and that the most recent version of the Markua Manual (https://leanpub.com/markua/read) currently documents Markua 0.1, not Markua 0.30.
Now, the good thing about the Markua spec is that even though it is long, it is very well-organized: since it starts from the CommonMark spec, which is based on the original Markdown spec, I'm standing on the shoulders of giants.
So, you can quickly find out, say, how to add index entries without reading the whole spec.
Now, it would be nice to have a quick reference to Markua. There is an example-based one in the spec itself, here:
Beyond that, probably the best quick reference is the original Markdown spec itself:
The example-based version is here:
(These are both from the original Markdown spec, which is so short it's actually a quick reference. The CommonMark spec (https://spec.commonmark.org/0.30/) which is based on Markdown is obviously way more complete, but it's so long that if the original Markdown spec had been that spec, then, in my opinion, Markdown probably would not have achieved the prominence it has today!)
Now, in terms of what you're looking for, which is to easily try stuff in Markdown / Markua and see the output, there are two good ways to go:
1. Download a Markdown preview plugin for VSCode. For example, I'm pasting a screenshot from Markdown Preview Enhanced below. This is by far the best way to go.
2. Use the CommonMark "dingus" thing: https://spec.commonmark.org/dingus/
Now, both of these tools are for Markdown, not Markua, but Markua is basically Markdown minus inline HTML plus some extensions. So, since anything which is Markua-specific will just get treated as text by a competent Markdown previewer, that will be fine. Maybe someday we'll write a Markua previewer for VSCode, but that won't be in 2022, and probably not in 2023 either!