Remove redundant operational sections from core style guide

Description

Removing the following redundancies from gitlab-translation-styleguide.md. These are operational instructions that are fully or mostly covered by the system prompt (steps 1-3): system-prompt.md

• Directory Structure is fully redundant. System prompt step 2 covers all locale mappings, directory paths, and the es not es-es callout with full URL examples. Removing entirely.
• Incremental Updates is mostly redundant. "Check if file exists" is step 1, "preserve existing translations" is in step 3. Two pieces not yet in system prompt - "remove deleted content" guidance and "when to update existing translations" criteria. They belong in the "Update Existing Translations" workflow being introduced in Implement new translation workflow and restruct... (#29). Removing from style guide now; carryover will be tracked in #29.
• Workflow (7-step list) is fully redundant. This is a compressed restatement of system prompt steps 1-5. Removing entirely.
• Blog Translation Rules , keeping for now. Blog-specific content rules (keep English filenames, don't translate category/tags, translate title/content/alt text) sdeem legitimate style guidance, not workflow. Will review later

Related

• Related to #20 (closed)

Type of Change

• New feature
• Bug fix
• Documentation update
• Style guide or terminology update
• Configuration change
• Other (please describe)

Changelog Entry

• Changelog entry added to CHANGELOG.md
• Entry is in the [Unreleased] section
• Used appropriate category (Added/Changed/Fixed/Deprecated/Removed/Security)
• Included MR reference at the end of the entry (e.g., (!123))