docs: update backend and pipeline integration instructions

[x0Lazarus]

The backend and pipeline integration section points readers to sigma/cli/backends.py and sigma/cli/pipelines.py, which no longer exist. Sigma CLI now discovers installed plugins through pySigma.

Replace the manual registration instructions with installation commands for pip, pipx and Poetry environments, link the pySigma plugin guide, and show how to check the discovered backend, pipeline and format identifiers.

Validation:

• python -m pytest -o addopts= -p no:cacheprovider tests/test_lists.py -q: 15 passed on Python 3.12.
• An offline synthetic plugin smoke check confirmed discovery through sigma.backends and sigma.pipelines, plus the list targets, list pipelines and list formats commands, with pySigma 1.5.0.
• git diff --check passed.