[Proposal] Add user guide, especially about directives

I noticed that there's no list for directives. In fact, there's not a comprehensive user guide in English (the only I found was in Japanese: https://docs.ruby-lang.org/ja/latest/library/rdoc.html).

Adding user guide has two purposes. First, of course, users will refer to the guide so that they can use directives and other features of RDoc more effectively. Second, it will be a single source of truth for RDoc behavior so developers can rely on it.

I suggest the first version of the guide could be a direct translation of the Japanese guide mentioned above since we don't have to talk about what should be in the guide.

Any thoughts?

Start by reviewing the Japanese RDoc guide linked in the issue, particularly its coverage of directives. Define the scope of an English user guide covering directives and other RDoc features, then translate and organize that material so it becomes a single reference for users and developers.