[mlir][doc] Fix links and indentation of mlir::ModuleOp description

All lines after the first are currently indented by one char further to the left than the first line. This leads to the first character of each sentence being cut from the resulting Markdown file after compilation. The text also contains 3 references to sections of other markdown files. One was missing the file, while the other two had outdated files, leading to 404 errors in the documentation.

Differential Revision: https://reviews.llvm.org/D102983