Improving the message about adding an implementation plan

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What does this MR do and why?

Issues

The current message about adding a chapter of the "implementation plan" is difficult to read. It does not even have quotes for the suggested names, and it is difficult to distinguish from the text.

When using the translator in the browser, things get even worse, since the suggested heading names are also translated (and the # characters in the text interfere with the translation). Since the bot only checks the heading names in English, they should not be displayed translated.

Changes

This MR adds quotes (single backticks) for example heading names (as in the handbook), and also simplifies the wording.

As far as I can see, three levels of headers are checked. I think in this case there is no point in specifying this in the comment, since it is unlikely that anyone will create a header of level 4 or higher.

Also, for clarity, a link to the Markdown documentation for headings has been added.

Expected impact & dry-runs

I just changed the text and did not test it in action. Please, someone make sure that the changes are correct.

Action items

• If adding environment variables for reactive processors, update config/triage-web.yaml and .gitlab/ci/triage-web.yml
• (If applicable) Add documentation to the handbook pages for Triage Operations =>
• (If applicable) Identify the affected groups and how to communicate to them:
  • /cc @person_or_group =>
  • Relevant Slack channels =>
  • Engineering week-in-review