message: docview: add mathematics in lean
diff: @@ -50,6 +50,7 @@ export class DocViewProvider implements Disposable {
  'Theorem Proving in Lean':
  'https://leanprover.github.io/theorem_proving_in_lean/',
  'Reference Manual': 'https://leanprover.github.io/reference/',
  + 'Mathematics in Lean': 'https://avigad.github.io/mathematics_in_lean/',
  };
  return '<ul>' +
  Object.getOwnPropertyNames(books).map((n) =>
  