Sheet.rowsAsMaps reads a worksheet as a List<Map<String, dynamic>>, taking
the keys from a header row. Every map holds a key for each column in the
used width, an empty header cell falls back to its column letter, and a
repeated name gets a _2 suffix, so no column is silently dropped.

Sheet.toJson serialises one sheet, either as header-keyed objects or, with
headerRow: null, as an array of arrays. Excel.toJson serialises the whole
workbook keyed by sheet name, or a single named sheet.

Values reuse the CSV bridge's scalar mapping, so numbers and booleans keep
their Dart types, dates and times become ISO-8601 strings, and a formula
exports its cached result. Adds a docs page at /excel-to-json.