Document how to embed code in the docs

[mdelapenya]

Instead of pasting code snippets, we usually embed code blocks with the <!--codeinclude--> block, which allows to have the docs in sync with the code.

It basically needs:

1. in the markdown:

<!--codeinclude-->
[Tab text](../path/to/file.extension) inside_block:name_of_the_block
<!--/codeinclude-->

2. in the file.extension

// name_of_the_block {
... code snippet to be rendered in the markdown
// }

Originally posted by @mdelapenya in #1970 (comment)