fix(docs): fix header custom button field names in theming guide (#25)

[cameo-js]

Description

This PR fixes incorrect field names in the theming customization documentation. The documentation was using customButtonLeft and customButtonRight, but the actual TypeScript interface uses leftAction and rightAction.

Changes

• Updated all instances of customButtonLeft to leftAction in the documentation
• Updated all instances of customButtonRight to rightAction in the documentation
• Aligned documentation examples with the actual TypeScript interface

Before

header: {
  customButtonLeft: {
    icon: "settings-cog",
    onClick: () => alert("Profile settings"),
  },
  customButtonRight: {
    icon: "home",
    onClick: () => openHomePage(),
  },
}

After

header: {
  leftAction: {
    icon: "settings-cog",
    onClick: () => alert("Profile settings"),
  },
  rightAction: {
    icon: "home",
    onClick: () => openHomePage(),
  },
}

Fixes #25

[weedon-openai]

Thank you for all these docs fixes!