Add shortcodes reference page for Academy content creators

Document the definitive list of all supported shortcodes available in the
Academy platform, organized by source: Academy theme (13 shortcodes),
Docsy theme (14 shortcodes), and Hugo built-in (7 shortcodes). Each
entry includes parameters, defaults, and usage examples.

Signed-off-by: Lee Calcote <lee.calcote@layer5.io>

Author: leecalcote