Include the file-extension with the image-customization.lua file. This
will ease work with editors providing syntax highlighting, as they now
properly detect the file-type.

Signed-off-by: David Bauer <mail@david-bauer.net>

Now we use the new Image-customization framework in Gluon, we need to
also update the docs so examples and descriptions are acurate again.

Signed-off-by: David Bauer <mail@david-bauer.net>

2 spaces is the most common indentation width used in the docs; adjust
the rest for consistency.

Also change .editorconfig accordingly.

This reverts commit 4e070312.

The recommendation is not sufficient at this time, so revert it.

See #1898 for further discussion.

This reverts commit b92e4044.

Committing in this case is unnecessary as that job is already handled by 998-commit
which is in a superior position to commit, as it is run after all relevant changes to
the configuration have been made.