docs: add comprehensive keybinds and custom themes documentation

[shatrughantwt]

Closes #38

Description

Fills in the two placeholder pages under 02_using_gitx with complete, example-driven documentation.

Changes:

02_key_binds.md

• Default keybindings reference (31 keybinds across 6 categories)
• Config file format, location, and key specification syntax
• Examples: vim, emacs, arrow keys, and Dvorak layouts

03_theme.md

• Built-in themes (GitHub Dark, Gruvbox) with full palettes
• Custom theme TOML format with 4 complete examples (Dracula, Solarized, Nord, One Dark)
• Usage instructions, color reference, and troubleshooting

All keybindings cross-referenced against keys.go, theme structure against theme.go, and config paths against config.go.

[bakayu]

These look great