Add CMake installation and find_package() support

* Alias the KleidiAI target for consistent linking to KleidiAI with `KleidiAI::kleidiai`.
* Add install instructions for the KleidiAI library target and its public headers.
* Export the KleidiAI target so that projects may use the imported target with `find_package()`.

Usage:

Install KleidiAI:
```
cmake -S . -B build
cmake --build build
cmake --install build
```

Once installed, KleidiAI can be imported and linked to using `find_package()`.

```
find_package(KleidiAI CONFIG REQUIRED)
target_link_libraries(my_framework PRIVATE KleidiAI::kleidiai)
```

Signed-off-by: Jakub Sujak <jakub.sujak@arm.com>

Approved-by: Viet-Hoa Do <viet-hoa.do@arm.com>