Document dbg, expect, in tutorial

See #12 (closed) for context. To improve the ergonomics of template debugging, we now have:

• dbg template option for dumping expansion (no need for cargo expand etc.)
• expect items etc. to get better syntax error reporting (IDE can "go to error" in expanded text)
• dbg_all_keywords, which dumps the expansions to help with choosing the right keyword (!97 (merged), !65 (closed)) #20 (comment 2961535)

These are documented in the relevant parts of the reference. They should be discussed at some suitable point(s) in the introduction.md.