feat: add function reference section to package pages using INDEX #124

[AdityaGupta716]

Closes #124

This PR adds a Function Reference section to each package page by parsing the INDEX files shipped with Octave packages.
Changes:

Added fetch_function.py script that fetches and parses INDEX files from each package's GitHub repository
Updated _layouts/package.html to display functions grouped by category
Updated 57 package YAML files with their function lists
Gracefully handles packages without INDEX files

[AdityaGupta716]

Hi @pr0m1th3as built a prototype in my fork Went with parsing the INDEX files from each package's repo — felt like the cleanest approach. Handles the cases where INDEX isn't available too. Plz review