docs: add specific gutenberg block id declaration to its page

[MrZyr0]

Following a problem loading my Gutenberg blocks with translated titles, I found the issue #872 which demonstrates the possibility of giving a fixed id independent of the translated title of the block.
As this is necessary for blocks with translated titles to remain functional when changing the language of a page edited with WPML, I propose to modify the example in the documentation to highlight this good practice :)