feat: add support for site-specific template files

[Sadashii]

This PR addresses #633 by introducing support for loading different templates depending on the competitive programming site/platform.

Features Added:

• cph.general.siteTemplates Configuration Setting: Adds an object setting in VS Code allowing users to map specific site names (case-insensitive keys like "USACO", "Codeforces", "AtCoder") to template file paths.
• Smart Matching: Evaluates the problem's group (problem.group) case-insensitively against the keys configured in siteTemplates.
• Graceful Fallbacks: If a site-specific template matches, it is loaded. If not, it falls back to the default language template configured under cph.general.defaultLanguageTemplateFileLocation.

Example Configuration:

"cph.general.siteTemplates": {
    "USACO": "F:\\templates\\usaco_file_io.cpp",
    "Codeforces": "F:\\templates\\codeforces_fast_io.cpp"
}

[agrawal-d]

Thanks! Please add more details (eg. how to configure in the setting description - I cant tell from reading the setting description how I would configure it for Codeforces and USACO as you show in the example.

And add even more details and screenshot(s) in the user-guide.

[agrawal-d]

Needs documentation