Reconsider AsciiDoc source file/folder names

The AsciiDoc source file/folder names now determine the website HTML URLs. Check all of them to ensure:

• They are nicely end-user readable URLs.
• It remains extensible. E.g. just 'tutorial' when we may get multiple tutorials in the future would lead to URL changes. We may be able to prevent some of them now.

We should do this before releasing the first multi-page HTML output.