docs: use larger headers for the suggestions docs, add a TOC (#1117)
docs: use larger headers for the suggestions docs, add a TOC (#1117)

Progress toward #1107

Since suggestions are the main unit of organization in the doc, it's useful for
those headings to be underlined and to be able to easily jump to any section.
eventualbuddha/decaffeinate,alangpierce/decaffeinate,alangpierce/decaffeinate,eventualbuddha/decaffeinate,decaffeinate/decaffeinate,decaffeinate/decaffeinate,alangpierce/decaffeinate