docs(intent): kind: statement - balance sheets and income statements

[delchev]

Documents kind: statement, added in eclipse-dirigible/dirigible#6912 (closes eclipse-dirigible/dirigible#6909).

A new subsection under reports in the Intent DSL reference, next to kind: balance:

• the shape — account: (the field holding the account code the lines select on) and lines:;
• the leaf vs computed split — accounts:+measure: reads the ledger, sum:/less: is arithmetic over other lines' codes;
• the selector grammar as a table — 20* prefix, 4110 exact, 60-69 a range over equally long code prefixes (which is why the bounds must be the same length: a comparison of whole codes drops 601);
• the twelve measures, and a tip explaining why the Net ones net per account, before the line sums — a settlement account is an asset in debit and a liability in credit, so pairing closingNetDebit on the asset line with closingNetCredit on the liability line files each account on the side its actual balance puts it on. Netting after the sum would report the section's gross turnover instead;
• the rules, and the boundary: the statement's numbers are the model's, the legally mandated print layout stays a hand-authored print template (linked to /help/intent/printing).

npm run docs:build is clean; no em/en dashes; the one cross-link resolves to an existing page.