docs: Revise language in walkthrough pages

Description

This merge request is a post-merge review of the wording changes in !1787 (merged).

Adheres to latest guidance for style in:

• https://docs.gitlab.com/ee/development/documentation/styleguide/word_list.html#chat-gitlab-duo-chat
• https://docs.gitlab.com/ee/development/documentation/styleguide/word_list.html#code-generation
• https://docs.gitlab.com/ee/development/documentation/styleguide/word_list.html#code-suggestions

Related Issues

• Related to feat: add gitlab workflow walkthrough (!1787 - merged) which I didn't get to review before it merged.
• Related to [VS Code] Add Walkthrough to Setup Duo (#1395 - closed) which tracks the overall work to add a walkthrough

How has this been tested?

Screenshots (if appropriate)

What CHANGELOG entry will this MR create?

• fix: Bug fix fixes - a user-facing issue in production - included in changelog
• feature: New feature - a user-facing change which adds functionality - included in changelog
• BREAKING CHANGE: (fix or feature that would cause existing functionality to change) - should bump major version, mentioned in the changelog
• None - other non-user-facing changes