[mlir][doc] Trim summary text during DocGen (llvm#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_
```

Author: rikhuijzer

mlir/tools/mlir-tblgen/OpDocGen.cpp

@@ -54,8 +54,9 @@ using mlir::tblgen::Operator;
 
 void mlir::tblgen::emitSummary(StringRef summary, raw_ostream &os) {
   if (!summary.empty()) {
-    char first = std::toupper(summary.front());
-    llvm::StringRef rest = summary.drop_front();
+    llvm::StringRef trimmed = summary.trim();
+    char first = std::toupper(trimmed.front());
+    llvm::StringRef rest = trimmed.drop_front();
     os << "\n_" << first << rest << "_\n\n";
   }
 }