Adding 1.12 release notes to TOC

Author: VesaJuvonen

docs/toc.yml

@@ -426,6 +426,8 @@
     items:
     - name: Overview
       href: spfx/roadmap.md
+    - name: SPFx v1.12.0 - March 17, 2021
+      href: spfx/release-1.12.0.md
     - name: SPFx v1.11.0 - July 16, 2020
       href: spfx/release-1.11.0.md
     - name: SPFx v1.10.0 - January 7, 2020