Rebrand Carbon Java to Carbon Lang across documentation

[Copilot]

Updated all references from "Carbon Java" to "Carbon Lang" throughout the repository documentation and configuration files to reflect the official language name.

Changes

• Documentation files: Updated 19 files across docs/ directory with terminology changes
• Total replacements: 31 occurrences updated

Example

Before:

# The Basics of Carbon Java

Carbon Java is designed to be as simple as possible.
A Carbon mod is a `.zip` file you drop into your mods folder. Inside that zip are scripts written in Carbon Java...

After:

# The Basics of Carbon Lang

Carbon Lang is designed to be as simple as possible.
A Carbon mod is a `.zip` file you drop into your mods folder. Inside that zip are scripts written in Carbon Lang...