docs: add Installation and Personalization sections; fix wiki index

- docs/fish-config.md: add §10 Installation (clone, sentinel guard,
  config-update usage) and §11 Personalization (secrets.fish /
  local.fish examples); update TABLE OF CONTENTS
- docs/split-wiki.py: strip trailing thematic break from DESCRIPTION
  body so index.md no longer renders a double --- separator
- Regenerate docs/wiki/ with 11-installation.md and
  12-personalization.md; all nav bars updated

docs/wiki/index.md

@@ -41,8 +41,6 @@ The configuration is split across:
 
 ---
 
----
-
 ## Table of Contents
 
 - [1. Configuration Variables](1-configuration-variables.md)
@@ -54,3 +52,5 @@ The configuration is split across:
 - [7. Customization](7-customization.md)
 - [8. Fisher Plugins](8-fisher-plugins.md)
 - [9. Viewing This Manual](9-viewing-this-manual.md)
+- [10. Installation](10-installation.md)
+- [11. Personalization](11-personalization.md)