Rename pages, replace front page, add add glossary

This MR introduces changes to the structure of the pages:

1. Rename pages: Add order to pages (1_page_name.py) and consider different names to what would be better to show as page titles in the sidebar.
2. Move the front page to a playground folder temporarily.
3. As the old front page is failing, replace it with calephant_front_page.py
4. Add a glossary with definitions for common terms.