Add vercel.json for git-integrated docs deployment

Lets Vercel build the same mkdocs site from source on every push,
instead of a one-off manual upload of the generated site/ folder.

Author: parthmax2

vercel.json

@@ -0,0 +1,5 @@
+{
+  "buildCommand": "pip install mkdocs-material && mkdocs build --strict",
+  "outputDirectory": "site",
+  "framework": null
+}