Add community guides for pseudo branching and HW decode colorspace

[yajrendrag]

Summary

• Pseudo Branching in Library Processing: Guide for using two Unmanic libraries over the same media folder with complementary filtering plugins (limit_library_search_by_ffprobe_data and ignore_files_with_matching_ffprobe_data) to process files differently based on ffprobe metadata. Includes an HDR/SDR splitting example.
• Handling Colorspace Changes with Hardware Decode: Guide for resolving ffmpeg filter graph failures caused by colorspace changes when using HW-accelerated decoding (NVENC, QSV, VAAPI). Covers the Safe decode mode option and three approaches: manual reprocessing, always-on safe mode, and automated pseudo branching with the reprocess_file plugin.

Test plan

• Verify both guides render correctly on the Docusaurus site
• Confirm guide shields (Difficulty, Setup Time, Original Author) display properly
• Check all links resolve correctly

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