I've posted this question on Bitcoin.org GitHub repository and I've got the answer.
According to @harding, from Github, the content of Bitcoin.org is commonly licensed under MIT, only not when specified. Here is his full answer:
the MIT license, as stated. You can also check the COPYING file in the repository, which says;
The intellectual property rights in the files are owned by the respective authors. Some of the files can be licensed under MIT License (MIT) available on http://opensource.org/licenses/MIT or other licenses. Appropriate licensing information can be found in the header of the file or in the folder containing the file.
In general, text files have the license information near the top and image files have the license information in a file in the directory.
[FAQ] certain pictures listed on the page are not MIT licensed (see the COPYING file here: https://github.com/bitcoin/bitcoin.org/tree/master/img/faq). This being said, although I am not a lawyer, I think you can easily use screenshots like this without much copyright issues
Thanks for your replies, guys, but I will elect this one as the best answer cause this is the response I really wanted.