Proper debugging advice

[pavpanchekha]

Each chapter should have comprehensive debugging advice. For example:

• For networking, advice to print the request and response; to trying latin1; to compare with curl; headers to look out for like Transfer-Encoding
• For graphics, instructions on installing Tk and quieting the macOS deprecation warning, and listing out what we actually need from a GUI framework in case you're working on another language or platform
• For text, instructions on printing the baseline, ascents, and descents; on flushing the line buffer; on printing the state at each character in the lexer
• For HTML, instructions on printing the open tags at each token