add a callout about header IDs

*   sub-headings

*   HTML layout (e.g., `div` elements).

> ## Linking section IDs 

>

> In the HTML output each header of a section, code sample, exercise will be associated with an unique ID (the rules of 

> the ID generation are given in kramdown [documentation](https://kramdown.gettalong.org/converter/html.html#auto-ids),

> but it is easier to look for them directly in the page sources). 

> These IDs can be used to easily link to the section by attaching the hash (`#`) followed by the ID to the page's URL 

> (like [this](#linking-section-ids)). For example, the instructor might copy the link to 

> the etherpad, so that the lesson opens in learners' web browser directly at the right spot. 

{: .callout}

## Formatting Code

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