feat: add Document::page(int $n) to look up a page by number

[impruthvi]

Summary

• Adds Document::page(int $number): Page — looks up a page by its 1-based page number
• Adds Document::hasPage(int $number): bool — non-throwing existence check, so callers can probe without a try/catch
• Throws PageNotFoundException (new, extends ParselException) when the number doesn't exist in the document
• Gives callers a typed, descriptive exception instead of an Undefined array key error from direct array access
• Both lookups share a private find() helper; page() is annotated with @throws

Usage

$document = Parsel::file('report.pdf')->parse();

if ($document->hasPage(3)) {
    $page = $document->page(3); // returns Page, guaranteed present
}

$document->page(99); // throws PageNotFoundException

Test plan

• Returns correct Page instance for a valid page number
• hasPage() returns true for an existing page and false for a missing one
• Throws PageNotFoundException with message "Page 99 was not found in the document." for missing page
• Lint (pint + rector), PHPStan, and unit suite pass