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/split-wiki.py

@@ -92,7 +92,9 @@ def main() -> None:
             numbered_sections.append(s)
 
     # ── Write index.md ──────────────────────────────────────────────────────
-    intro_body = "\n\n".join(intro_parts)
+    intro_body = "\n\n".join(intro_parts).rstrip()
+    # Strip any trailing thematic break that the source adds before the ToC.
+    intro_body = re.sub(r"\n+---\s*$", "", intro_body)
     toc = build_full_toc(numbered_sections)
     index_content = f"# Fish Shell Configuration\n\n{intro_body}\n\n---\n\n{toc}\n"
     (out_dir / "index.md").write_text(index_content)