Updated Home/Get started/Fundamentals/Additional features to reflect MPAv2

[treuille]

📚 Context

The Streamlit framework now uses the st.Page and st.navigation APIs
for creating multipage apps. This PR updates our documentation to reflect
these changes.

🧠 Description of Changes

• Updated additional-features.md documentation to use new multipage APIs
• Changed file structure documentation to show:
  • streamlit_app.py as the entry point/router
  • main_page.py, page_2.py, and page_3.py as content files
• Added new code examples demonstrating:
  • Page definition using st.Page()
  • Navigation setup with st.navigation()
  • Page execution flow
• Updated command examples from streamlit run main_page.py to streamlit run streamlit_app.py
• Replaced screenshot mpa-main-concepts-2.gif with mpa-v2-main-concepts.gif

🧠 Description of NON-Changes

• Mulitpage apps are also described in "Home/ Get started/ First steps/ Create a multipage app."
• This PR does not update that part of the docs yet, which should be updated
  indepenently.
  • Let me know, and I can do that one too!

Revised:

This is the new documentation:

Current:

This is the old documentation:

💥 Impact

Size:

• Small
• Not small

🌐 References

No references.

Contribution License Agreement

By submitting this pull request you agree that all contributions to this project are made under the Apache 2.0 license.