Expand blog translation rules for GDATP
**Description** As part of #20+ (core style guide restructuring), the Blog Translation Rules section will be from the core style guide into its own file: `specifications/core/blog-translation-rules.md`. Link to current _Translation rules for marketing blogs_: https://gitlab.com/gitlab-com/localization/gitlab-duo-agent-translation-platform/-/blob/main/specifications/core/website-translation-rules.md?ref_type=heads#translation-rules-for-marketing-blogs The Content Management team is using GDATP for blog translation, [but less than AIQT](https://gitlab.com/gitlab-com/localization/content-management/-/work_items?sort=created_date&state=opened&search=AIQT&first_page_size=100) https://gitlab.com/gitlab-com/localization/localization-team/-/work_items/424, due to lower content quality. Goal is to improve the style guide and glossary, so GitLab Translation Agent / GDATP can be used for blog post localization
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