Restructure core style guide to remove redundancy with system prompt

Core style guide: https://gitlab.com/gitlab-com/localization/gitlab-duo-agent-translation-platform/-/blob/main/specifications/core/gitlab-translation-styleguide.md

The gitlab-translation-styleguide.md file mixes several different types of content that should ideally be separated, for example

• a) style guidance (which belongs to the guide on style conceptually),
• b) workflow instructions (which rather belong in the system prompt),
• c) operational procedures about MR creation, or directory structure (which I am not sure where should belong, maybe system prompt).

After this work / success criteria

• Core style guide will contain only relevant style guidance (TBD what it is, maybe SEO limits, what to translate / never translate)
• System prompt will contain workflow instructions and operational procedures (MR creation, directory mapping, validation checklists)
• No duplicated content between the two files