Simplify character token representation

[gkellogg]

As suggested in https://github.com/w3c/rdf-turtle/pull/39#issuecomment-1738514256.

• Gather explicit token representations with code points in a "How to Read this Document" section.
• Change existing explicit code point annotations with references into the "How to Read this Document" section.
• Use tooltip (attribute title="") on tokens to make it clear which Unicode character is used.
• Add a sentence that the grammar and tokens are expressed using the ASCII range of Unicode.