docs: reorganize upgrade documentation structure

- Rename docs/upgrading-spec-kit.md to docs/upgrade.md for brevity
- Update README and documentation index links to point to new upgrade guide
- Update table of contents to use shorter "Upgrade" title
- Simplify section headers in upgrade guide
- Add Getting Started section back to docs/index.md with upgrade guide link

Commit-generated-by: GitHub Copilot <github.copilot@github.com>
Signed-off-by: Ashley Childress <6563688+anchildress1@users.noreply.github.com>

docs/toc.yml

@@ -9,8 +9,8 @@
       href: installation.md
     - name: Quick Start
       href: quickstart.md
-    - name: Upgrading Spec Kit
-      href: upgrading-spec-kit.md
+    - name: Upgrade
+      href: upgrade.md
 
 # Development workflows
 - name: Development