Update blogs and categories

[sachajudd]

https://userhelp.silverstripe.org/en/4/optional_features/blogs/tags-and-categories
Getting quite a lot of negative feedback about the blog module and how it's not clearly documented.

"Explain where date and author are entered. They are impossible to find!"
"Should display an indication of how to enable the feature"
"How can I get a post by tag?"