[mlir][Linalg] Fix formatting of generated docs markdown

This patch prevents `mlir-linalg-ods-yaml-gen` from adding extra
whitespace around the summary and description fields. This broke the
_italics_ of the summary as _ this _ is not recognised by markdown.
It also meant the first line of the description was in a code block
  as it was indented two spaces.

The separator between summary and description has also been updated to
two newlines. This was already followed and prevents line wrapping the
summary putting part of it in the description.

These issues can be currently seen at: https://mlir.llvm.org/docs/Dialects/Linalg/

Reviewed By: awarzynski

Differential Revision: https://reviews.llvm.org/D157853