Fix lack of documentation on how to fetch a snippet's content using API

What does this MR do?

Dear Gitlab maintainers,

I spent a bit more time than I would have liked to when searching for a way to access snippets' contents via the API. This little patch adds the missing documentation about the /raw endpoint that does exactly what I hoped it would do.

Hope this helps, Colin

Related issues

I have not found a related issue.

Author's checklist

(I think those don't apply for such a little fix?)

• Apply the correct labels and milestone
• Crosslink the document from the higher-level index
• Crosslink the document from other subject-related docs
• Feature moving tiers? Make sure the change is also reflected in features.yml
• Correctly apply the product badges and tiers
• Port the MR to EE (or backport from CE): always recommended, required when the ee-compat-check job fails

Review checklist

• Your team's review (required)
• PM's review (recommended, but not a blocker)
• Technical writer's review (required)
• Merge the EE-MR first, CE-MR afterwards