docs: add submodule for Ovum documentation and remove outdated files

- Introduced a new submodule for the Ovum documentation repository.
- Deleted outdated documentation files including BUILD.md, README.md, and various reference materials to streamline the documentation structure.

Author: bialger

.gitmodules

@@ -0,0 +1,4 @@
+[submodule "docs"]
+	path = docs
+	url = https://github.com/Ovum-Programming-Language/OvumDocs.git
+	branch = main