i18n: document how to use links without splitting sentences

What does this MR do?

Documents how to use links in translated strings without splitting sentences.

We have various implementations of using links in translated strings and have determined that the best practice is set the beginning and ending fragments of a link as variables, so that the string itself still retains the full context.

One instance of this discussion of this can be found at https://gitlab.com/gitlab-org/gitlab-ce/merge_requests/19495#note_79545331

Does this MR meet the acceptance criteria?

• Documentation created/updated
• Conform by the code review guidelines
  • Has been reviewed by a Frontend maintainer
  • Has been reviewed by a Backend maintainer
• Conform by the style guides
• End-to-end tests pass (package-and-qa manual pipeline job)