Consolidate Never Translate into single source of truth and rename core style guide

Description

This MR merges the "Never Translate" list from system prompt , and two overlapping sections from the core style guide ("Never Translate" + "Technical Content Rules"), into a single consolidated "Never Translate" section in the website-translation-rules file.

Full list of changes

• Renamed from gitlab-translation-styleguide.md to website-translation-rules.md to reflect what the file actually contains after restructuring
• Merged "Never Translate" and "Technical Content Rules" into one "Never Translate" section
• Removed "Technical Content Rules" section , was absorbed into "Never Translate"
• Removed "Quality Checklist" section (before/during/after) , because it is redundant or human-oriented
• Added HTML comment on Blog Translation Rules, with intent to eventually move to its own file for content team to iterate on

System prompt changes

• Updated core resource URL in step 2 from gitlab-translation-styleguide.md to website-translation-rules.md
• Removed "Never translate" list and replaced with reference to full URL of website-translation-rules.md
• Moved preserve-existing-values guidance (formId: 3240 example) into the "Important" note in step 3. Reason: this is more of a workflow instruction, than a "never translate" rule

Result 🎉

• One source of truth for all "never translate" rules in website-translation-rules.md
• System prompt references it instead of maintaining a separate list

To-do

• Update README accordingly

Related

• Related to #20 (closed)

Type of Change

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

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))