[mlir] Add documentation for TableGen LSP features and setup

This commit beefs up the documentation for MLIR language servers by
adding proper documentations/examples/etc for the provided TableGen
language server capabilities. Given that this documentation is also used
for the vscode extension, this commit also updates the user facing vscode
extension documentation.

Note that the images referenced in the new documentation are hosted on
the website, and will be commited to mlir-www shortly after this commit
lands.