docs: Update GEPs in navbar (#3634)

This commit updates the list of GEPs displayed in the site's navigation
sidebar to add missing GEPs, or move GEPs to accurately reflect their
current state.

**Added**

* GEP-1767: Provisional
* GEP-3567: Implementable

**Moved**

* GEP-91: Provisional -> Implementable
* GEP-1867: Provisional -> Standard
* GEP-2649: Provisional -> Experimental
* GEP-3155: Implementable -> Experimental
* GEP-3388: Provisional -> Implementable

Author: blake

mkdocs.yml

@@ -122,23 +122,23 @@ nav:
   - Enhancements:
     - Overview: geps/overview.md
     - Provisional:
-      - geps/gep-91/index.md
       - geps/gep-1494/index.md
       - geps/gep-1651/index.md
       - geps/gep-1713/index.md
-      - geps/gep-1867/index.md
+      - geps/gep-1767/index.md
       - geps/gep-2648/index.md
-      - geps/gep-2649/index.md
-      - geps/gep-3388/index.md
     - Implementable:
-      - geps/gep-3155/index.md
+      - geps/gep-91/index.md
+      - geps/gep-3567/index.md
+      - geps/gep-3388/index.md
     - Experimental:
       - geps/gep-995/index.md
       - geps/gep-1619/index.md
       - geps/gep-1731/index.md
       - geps/gep-1748/index.md
       - geps/gep-1897/index.md
       - geps/gep-2162/index.md
+      - geps/gep-2649/index.md
       - geps/gep-3155/index.md
       - geps/gep-3171/index.md
     - Standard:
@@ -158,6 +158,7 @@ nav:
       - geps/gep-1709/index.md
       - geps/gep-1742/index.md
       - geps/gep-1762/index.md
+      - geps/gep-1867/index.md
       - geps/gep-1911/index.md
       - geps/gep-2257/index.md
     - Memorandum: