docs(install): add source-aligned build and backend guide

Document installation workflows for llama-cpp-python with a focus on
the underlying llama.cpp CMake build configuration.

- Add virtual environment, source install, editable install, rebuild, and
verification guidance.
- Document common CMake options such as GGML_NATIVE,
  GGML_BACKEND_DL, GGML_CPU_ALL_VARIANTS, and compiler selection.
- Summarize backend-specific build flags for CUDA, BLAS, Metal, Vulkan,
  OpenVINO, HIP, SYCL, OpenCL, CANN, ZenDNN, and zDNN.
- Include backend runtime notes and common installation pitfalls while
  keeping server-related installation content out of the page.

Signed-off-by: JamePeng <[email protected]>

Author: JamePeng