[mlir][doc] Trim summary text during DocGen (#68477)

When defining a multi-line string in tblgen, the output in the Markdown
file currently contains too much whitespace and newlines for Hugo's
Markdown parser. For example, for `arith.addui_extended` the tblgen
```tblgen
let summary = [{
  extended unsigned integer addition operation returning sum and overflow bit
}];
```
is currently converted to
```markdown
_
    extended unsigned integer addition operation returning sum and overflow bit
  _
```
which causes the text to not be italicized (as can be seen at
https://mlir.llvm.org/docs/Dialects/ArithOps/#arithaddui_extended-arithadduiextendedop).
After this PR, the output becomes
```
_Extended unsigned integer addition operation returning sum and overflow bit_
```