docs: document conda-forge installation

[radixun]

Summary

• Document the conda-forge package and its explicit channel installation command.
• Keep the root Markdown README and generated RST counterpart consistent while preserving existing PyPI core/full instructions.

Follows up on the maintainer-requested README update in #120. The conda-forge package was already provided by its feedstock contributors; this change only documents it.

Validation

• The identical source-bound documentation check fails on the original commit because the conda route is absent, and passes on this committed documentation change in the offline isolated LinuxTest runner.
• Checks cover the actual installation sections, explicit channel/package command, official package reference, preserved PyPI guidance, and Markdown/RST command parity.
• Exact Git source/HEAD and process/source cleanup verified for both runs; git diff --check passes.

Conda installation, dependency solving, native API tests and Sphinx generation were not run. The RST addition was matched manually to the existing conversion style.