Remove hint of the Core Concepts pages

[rmartinezduque]

All the pages in the Core Concepts section have the same final hint ("Still have questions? Ask the community..."). We mention the community forum many times in the docs, so I think this hint is no longer necessary. If you still want to maintain it though, I would suggest replacing it with something like this:

If you still have any questions about Frontity, please check out the community forum. It is packed full of answers and solutions to all sorts of Frontity questions.