fix: Formatting issues / minor erros in the docs

[e1Ru1o]

Description

Suggestions of minor changes in docs

Problem

1. Some code section was not properly formatted due to missing ``` in quick start page
2. An extra word for in section of quick start page
3. push_front says to return an array
4. pop_front & pop_back say to return arrays

Summary

1. Added the missing ```
2. Removed the extra word
3. Changed to slice
4. Changed to slice

Additional Context

Documentation

Check one:

• No documentation needed.
• Documentation included in this PR.
• [For Experimental Features] Documentation to be submitted in a separate PR.

PR Checklist

• I have tested the changes locally.
• I have formatted the changes with Prettier and/or cargo fmt on default settings.

[github-actions]

Thank you for your contribution to the Noir language.

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