feat(qt): add build directory to gitignore

[add-uos]

Reasons for making this change

Qt projects commonly use a build/ directory for out-of-source builds, especially when using CMake (the default build system for Qt 6). While Qt.gitignore already includes *build-* to handle Qt Creator's build directories (e.g., build-MyProject-Debug), it doesn't cover the standard build/ directory that many Qt projects use.

This change applies to all Qt developers because:

1. Qt 6 uses CMake by default, which commonly generates a build/ directory
2. The official Qt documentation and examples often use build/ as the build directory
3. Many Qt tutorials and best practices recommend using a build/ directory for out-of-source builds
4. This aligns with C++.gitignore, which also includes build/

Links to documentation supporting these rule changes

• Qt 6 Documentation: CMake Manual - Shows examples using build/ directory
• Qt Wiki: Building Qt 5 from Git - Recommends using a separate build directory
• Qt Creator Manual: Build Settings - Documents how Qt Creator creates build directories
• CMake Documentation: CMake Tutorial - Standard practice is to use a build/ directory

Related .gitignore files:

• C++.gitignore (already includes build/)
• Python.gitignore (already includes build/)
• Most other language templates include build/ as it's a universal convention

If this is a new template

N/A - This is an improvement to an existing template

Merge and Approval Steps

• Confirm that you've read the contribution guidelines and ensured your PR aligns
• Ensure CI is passing
• Get a review and Approval from one of the maintainers