introduce code transformation to simplify writing ui.sub_pages demos

[rodja]

Motivation

Writing demos for Single Page Applications (SPAs) with ui.sub_pages has been cumbersome and unnatural. Demo code doesn't match what users should actually write in their own applications:

1. Unnatural code patterns: Demo code used FakeSubPages and manual workarounds (sub_pages.link(), FakeArguments, etc.) instead of the clean, straightforward API (ui.link(), ui.sub_pages())
2. Quite some code duplications which makes it hard to find areas where the rendered demo didn't match what users should write

Implementation

This PR introduces an automatic code transformation system that allows demo authors to write natural, idiomatic NiceGUI code while still supporting browser-based demo execution:

1. New transformation infrastructure (website/documentation/code_extraction.py):
   • Transform ui.sub_pages() → FakeSubPages.init(), ui.link() → pages.link()
   • Handles nested sub-pages, URL parameters, query strings, and chained methods
   • Uses special # TRANSFORM comments for complex cases (e.g., parameterized routes)
   • test cases covering transformation of sub-pages, nested routes, URL parameters, TRANSFORM hints
2. Demo execution engine (website/documentation/demo.py):
   • Displays non-transformed code block
   • Executes transformed code with automatic root() function detection
3. Updated sub-pages documentation (website/documentation/content/sub_pages_documentation.py):
   • All demos now use natural ui.link() and ui.sub_pages() API
   • Removed ~140 lines of workaround code

Progress

• I chose a meaningful title that completes the sentence: "If applied, this PR will..."
• The implementation is complete.
  • The transformation itself is AI generated hand I have not yet found the time to clean it up
• Pytests have been added (or are not necessary).
• Documentation has been updated.

[rodja]

While I generally like the idea I'm not sure the actual implementation is worth it. I guess it depends on how many ui.sub_pages tests we want to write/maintain.

[evnchn]

Prefer not to add code transformation to all of the demos just for ui.sub_page demos because we can't really be certain our new code transformer is the same as the old one if it is not a ui.sub_page demo.

How about we can pass in a transformer: Callable[[str], str] somewhere?