**Add dedicated CPU-only Dockerfile and update documentation for CPU/…

[jakubgajski]

…GPU builds**

What does this PR do?

NO CHANGE IN CODE

Introduces Dockerfile_llamacpp_cpuonly as a leaner alternative for llama.cpp backed inference than already existing Dockerfile_llamacpp (this: 1.7GB, existing: ~17GB)

This introduction targets users that do not need all the Nvidia related bloat when doing inference on CPU-only. Based on libopenblas.

Includes update of documentation.

Tested locally on Apple Silicon and AMD CPUs.

Fixes # (issue)

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