Let people select which sections to answer at start of survey

We could have a table of contents at the start of the entire survey that lets people uncheck sections they don't intend to answers or are not relevant to them, to save them time.

Start by locating the survey entry point and the code that renders the full section flow. Trace how sections are defined and selected, then clarify how skipped sections should behave before implementing the table of contents; done means respondents can exclude irrelevant sections and complete the remaining survey.

Written by the indexing model from the issue text.