feat(markdown): document new option langAlias

[ematipico]

Description (required)

Adds documentation for a new shiki option called langAlias

For Astro version: 4.16.0. See astro PR #12039.

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Tracked Files

Locale	File	Note
en	guides/markdown-content.mdx	Source changed, localizations will be marked as outdated.

Warnings reference

Icon	Description
🔄️	The source for this localization has been updated since the creation of this pull request, make sure all changes in the source have been applied.

[sarah11918]

More options are always great! See my suggestion below!

[sarah11918]

Noting that this PR looks good, but there might be something in the upstream PR re: whether or not langs is required. And if so, that will need to go here too. (If not, then this PR is fine!)