docs: add configuration documentation WIP

docs/SUBTITLE_CONFIG.md

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+# Subtitle Processing Configuration
+
+This document covers subtitle processing and formatting options.
+
+## subtitle (dict)
+
+Control subtitle conversion and SDH (hearing-impaired) stripping behavior.
+
+- `conversion_method`: How to convert subtitles between formats. Default: `pysubs2`.
+  - `auto`: Use subby for WebVTT/SAMI, standard for others.
+  - `subby`: Always use subby with CommonIssuesFixer.
+  - `subtitleedit`: Prefer SubtitleEdit when available; otherwise fallback to standard conversion.
+  - `pycaption`: Use only the pycaption library (no SubtitleEdit, no subby).
+  - `pysubs2`: Use pysubs2 library (supports SRT, SSA, ASS, WebVTT, TTML, SAMI, MicroDVD, MPL2, TMP formats).
+
+- `sdh_method`: How to strip SDH cues. Default: `auto`.
+  - `auto`: Try subby for SRT first, then SubtitleEdit, then filter-subs.
+  - `subby`: Use subby's SDHStripper. **Note:** Only works with SRT files; other formats will fall back to alternative methods.
+  - `subtitleedit`: Use SubtitleEdit's RemoveTextForHI when available.
+  - `filter-subs`: Use the subtitle-filter library.
+
+- `strip_sdh`: Enable/disable automatic SDH (hearing-impaired) cue stripping. Default: `true`.
+
+- `convert_before_strip`: When using `filter-subs` SDH method, automatically convert subtitles to SRT format first for better compatibility. Default: `true`.
+
+- `preserve_formatting`: Keep original subtitle tags and positioning during conversion. Default: `true`.
+
+Example:
+
+```yaml
+subtitle:
+  conversion_method: pysubs2
+  sdh_method: auto
+  strip_sdh: true
+  convert_before_strip: true
+  preserve_formatting: true
+```
+
+---