docs: Don't use section-index plugin during klipper3d.org site generation

The section-index plugin can cause some confusing navigation.  Use the
simpler site navigation for now.

Signed-off-by: Kevin O'Connor <kevin@koconnor.net>

docs/_klipper3d/mkdocs.yml

@@ -14,14 +14,12 @@ markdown_extensions:
   - toc:
       permalink: True
       toc_depth: 6
-  #- pymdownx.snippets
   - attr_list
   - mdx_partial_gfm
   - mdx_truly_sane_lists
   - mdx_breakless_lists
 plugins:
   - search
-  - section-index
   - mkdocs-simple-hooks:
       hooks:
         on_page_markdown: "docs._klipper3d.mkdocs_hooks:transform"
@@ -72,20 +70,19 @@ extra:
 
 # Navigation hierarchy (this should mimic the layout of Overview.md)
 nav:
-  - Overview:
-    - Overview.md
-    - Features.md
-    - FAQ.md
-    - Releases.md
-    - Config_Changes.md
-    - Contact.md
-  - Configuration and Tuning:
+  - Overview.md
+  - Features.md
+  - FAQ.md
+  - Releases.md
+  - Config_Changes.md
+  - Contact.md
+  - Installation and Configuration:
     - Installation.md
-    - Configuration reference:
+    - Configuration Reference:
       - Config_Reference.md
       - Rotation_Distance.md
     - Config_checks.md
-    - Bed level:
+    - Bed Level:
       - Bed_Level.md
       - Delta_Calibrate.md
       - Probe_Calibrate.md
@@ -93,7 +90,7 @@ nav:
       - Manual_Level.md
       - Bed_Mesh.md
       - Endstop_Phase.md
-    - Resonance compensation:
+    - Resonance Compensation:
       - Resonance_Compensation.md
       - Measuring_Resonances.md
     - Pressure_Advance.md
@@ -105,7 +102,7 @@ nav:
     - skew_correction.md
     - Using_PWM_Tools.md
     - G-Codes.md
-  - Developer documentation:
+  - Developer Documentation:
     - Code_Overview.md
     - Kinematics.md
     - Protocol.md
@@ -116,7 +113,7 @@ nav:
     - Benchmarks.md
     - CONTRIBUTING.md
     - Packaging.md
-  - Device specific documents:
+  - Device Specific Documents:
     - Example_Configs.md
     - SDCard_Updates.md
     - RPi_microcontroller.md