Update Copy in AI Features Page

What does this MR do and why?

Updates copy in the AI Features pages:

• Updates instances of "Code Generations" and "Code Completions" to use sentence case in line with GitLab style guides and singularized (as requested by technical writing team).
• "...empowered by a GitLab AI Vendor" -> "...powered by a GitLab AI Vendor"
• Other minor copy updates as outlined in the MR discussions

MR acceptance checklist

Please evaluate this MR against the MR acceptance checklist. It helps you analyze changes to reduce risks in quality, performance, reliability, security, and maintainability.

Screenshots or screen recordings

Screenshots are required for UI changes, and strongly recommended for all other merge requests.

Features page

Before	After

Feature config page

Before	After

How to set up and validate locally

Numbered steps to set up and validate the change are strongly suggested.