chore: change vue -> markup for syntax highlighting

[bsmth]

Description

This PR changes instances of vue to markup in code blocks.

```vue
// this is not a detected language
```


```markup
// loaded by Prism
```

Motivation

We have build errors that vue is not a detected language. There is no support for vue in Prism, an alternative is to use markup which is supported without additional Yari dependencies - this is acceptable for the 4 instances of vue code blocks IMO.

Related issues and pull requests

• Support Vue SFC syntax PrismJS/prism#1665 (comment)

[github-actions]

Preview URLs

• /en-US/docs/Learn/Tools_and_testing/Client-side_JavaScript_frameworks/Vue_computed_properties

[Josh-Cena]

I see no point to change the language meta. Prettier supports formatting for Vue, and we may even lint these code blocks one day. Can we add a mapping in Yari?

[bsmth]

Can we add a mapping in Yari?

Indeed we could add an alias for vue resolving to markup for the syntax highlighting part.