Alphabetize themes in code-only Themes doc (#3526)

Reorder the eight per-theme subsections under "Available themes" alphabetically (Bubblegum, DarkPro, Editor, Forest Glade, Hazard, Meadow, Neon, Retro95) so a theme is easier to find as the catalog grows. Pure block reorder — content of each block is unchanged.

Also update the top preview-grid caption to say "six of Gum's eight built-in themes" so it's clear the six showcased themes aren't the full set.

Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>

Author: vchelaru