docs: fix typos, brand capitalization, and grammar across READMEs

[umyunsang]

This PR fixes typos, grammar, and brand capitalization across documentation files.

Changes:

• README.md: Stable Diffusion capitalization; hyphenation; NVIDIA/PyTorch/ROCm brand fixes; Apple silicon wording; grammar and environment variable phrasing; Discord/open-source phrasing
• comfy_api_nodes/README.md: subject-verb agreement; missing verb; code block command fix (python main.py); argument formatting; inclusive terminology; Pydantic data models phrasing; YAML capitalization
• api_server/routes/internal/README.md: add missing conjunction for clarity
• tests/README.md: add missing pip install prefixes

Summary

This PR improves the overall readability and professionalism of the documentation by correcting various grammatical errors, fixing brand name capitalization, and ensuring consistent terminology throughout the README files.

I'd be happy to continue contributing documentation improvements to help make ComfyUI's docs even better for the community.