docs: add documentation comments for apps and paging types

- add doc comments to `AppsEndpoint`, `App` struct, and the `Get` method
in `apps.go`.
- add doc comments to the `Page` struct in `page.go`.
- add doc comments to `PagingRequest`, `PagingOption`, and paging helper
functions in `pagingRequest.go`.

pagingRequest.go

@@ -2,11 +2,13 @@ package onspring
 
 import "strconv"
 
+// PagingRequest contains pagination parameters for API requests.
 type PagingRequest struct {
 	pageNumber int
 	pageSize   int
 }
 
+// ToParams converts the paging request to a map of query parameters.
 func (pr *PagingRequest) ToParams() map[string]string {
 	return map[string]string{
 		"pageNumber": strconv.Itoa(pr.pageNumber),
@@ -14,14 +16,17 @@ func (pr *PagingRequest) ToParams() map[string]string {
 	}
 }
 
+// PagingOption is a function that modifies a PagingRequest.
 type PagingOption func(*PagingRequest)
 
+// ForPageNumber sets the page number for a paging request.
 func ForPageNumber(pageNumber int) PagingOption {
 	return func(pr *PagingRequest) {
 		pr.pageNumber = pageNumber
 	}
 }
 
+// WithPageSize sets the page size for a paging request.
 func WithPageSize(pageSize int) PagingOption {
 	return func(pr *PagingRequest) {
 		pr.pageSize = pageSize