Add notes on when to not translate

What does this MR do?

The user/markdown.md file has special translation rules to make it more usable after translation. In most cases, code is never translated, but in the Markdown examples we often show how markdown renders, which is translated. To make this work, we translate most code examples so things stay in sync.

But there are a few places where we use a screenshot instead of a rendered version, and in that case translating the code brings us out of sync. We have notes for translators for handling this page, but just as a secondary tool we should also add in-line HTML notes if someone is checking the source while translating.

For example:

• https://docs.gitlab.com/ja-jp/user/markdown/#heading-ids-and-links
• https://docs.gitlab.com/ja-jp/user/markdown/#inline-diff
• https://docs.gitlab.com/ja-jp/user/markdown/#alerts

Related issues

• Related to gitlab-com/localization/docs-site-localization#483 (comment 2728498212)

Author's checklist

• Optional. Consider taking the GitLab Technical Writing Fundamentals course.
• Follow the:
  • Documentation process.
  • Documentation guidelines.
  • Style Guide.
• If you're adding a new page, add the product availability details under the H1 topic title.
• If you are a GitLab team member, request a review based on:
  • The documentation page's metadata.
  • The associated Technical Writer.

If you are a GitLab team member and only adding documentation, do not add any of the following labels:

• ~"frontend"
• ~"backend"
• ~"type::bug"
• ~"database"

These labels cause the MR to be added to code verification QA issues.

Reviewer's checklist

Documentation-related MRs should be reviewed by a Technical Writer for a non-blocking review, based on Documentation Guidelines and the Style Guide.

If you aren't sure which tech writer to ask, use roulette or ask in the #docs Slack channel.

• If the content requires it, ensure the information is reviewed by a subject matter expert.
• Technical writer review items:
  • Ensure docs metadata is present and up-to-date.
  • Ensure the appropriate labels are added to this MR.
  • Ensure a release milestone is set.
  • If relevant to this MR, ensure content topic type principles are in use, including:
    • The headings should be something you'd do a Google search for. Instead of Default behavior, say something like Default behavior when you close an issue.
    • The headings (other than the page title) should be active. Instead of Configuring GDK, say something like Configure GDK.
    • Any task steps should be written as a numbered list.
    • If the content still needs to be edited for topic types, you can create a follow-up issue with the docs-technical-debt label.
• Review by assigned maintainer, who can always request/require the reviews above. Maintainer's review can occur before or after a technical writer review.