[i18n] Enforce explicit heading IDs

[chalin]

Originally posted by @chalin in #6134 (comment)

... Let's consider this error:

  hash does not exist --- es/docs/contributing/pr-checks/index.html --> ../style-guide/#file-names

It says that the file in the PR doesn't have an heading ID named file-names, which is true. For guidance, see:

• Translation guidance, in particular
• Heading anchors

Some things are beyond the current capabilities of our fix commands. For example, our fix commands don't add heading IDs at the moment (though I'm thinking of doing so as soon as I can). I hope this helps clarify what is going on.

Not having heading IDs created at the same time as new non-en pages are added seems to lead to confusion for some contributors, and sometimes leads to link failures later, when a heading ID becomes referred to for the first time in the en docs (and this gets propagated, via fallback pages) to the non-en docs.

Tasks

• Somehow enforce explicit heading IDs (maybe via a markdownlink rule) for non-en pages
• Update the Localization docs to reflect this requirement
• ...