feat(docs): add auto-generated Markdown wiki

Adds docs/split-wiki.py, a Python script that splits fish-config.md
into a multi-page Markdown wiki under docs/wiki/:

- index.md: DESCRIPTION intro + full section ToC
- one file per numbered section (1-configuration-variables.md … 9-viewing-this-manual.md)
- each section page has a nav bar at the top linking to every other section

The build-docs.yml workflow now runs the script alongside pandoc so all
three formats (man page, HTML, wiki) are regenerated together from a
single source in one CI run.

README slimmed down to a pointer to the wiki; docs/fish-config.md §9
gains an "As a wiki" subsection.

docs/wiki/7-customization.md

@@ -0,0 +1,40 @@
+# 7. CUSTOMIZATION
+
+**Sections:** [Index](index.md) | [1. Configuration Variables](1-configuration-variables.md) | [2. Path Setup](2-path-setup.md) | [3. Key Bindings](3-key-bindings.md) | [4. Abbreviations](4-abbreviations.md) | [5. Functions Reference](5-functions-reference.md) | [6. Dependency Catalog](6-dependency-catalog.md) | **7. Customization** | [8. Fisher Plugins](8-fisher-plugins.md) | [9. Viewing This Manual](9-viewing-this-manual.md)
+
+---
+
+## Machine-local Configuration
+
+Place machine-specific settings that should not be committed to git in:
+
+    ~/.config/.user-dots/fish/local.fish
+
+Typical uses: additional PATH entries, local aliases, hostname-specific env
+vars, work-specific tool configs.
+
+## Secrets and API Keys
+
+    ~/.config/.user-dots/fish/secrets.fish
+
+Store API tokens, GPG keys, private credentials here. This file is never
+committed.
+
+Both files are sourced at the end of config.fish on every interactive
+session, so they can override anything set earlier.
+
+## Overriding Configuration Variables
+
+Any variable set in local.fish after the main config loads takes effect.
+Example: to increase the scrollback history limit:
+
+    # in local.fish
+    set -gx SCROLLBACK_HISTORY_MAX_FILES 200
+
+## Fish Universal Variables
+
+Some settings (fzf colors, theme) are stored in fish_variables via
+`set -U`. These are machine-local and git-ignored. Do not commit
+fish_variables.
+
+---