Try README.md symlink instead of index.md symlink

It just copies it over, maybe it'll respect the symlink where Nginx or
mkdocs doesn't:
https://github.com/mkdocs/mkdocs/issues/321#issuecomment-70099532
diff --git a/docs/index.md b/docs/README.md
similarity index 100%
rename from docs/index.md
rename to docs/README.md