Add example and search term for 'repeat' to the fill command (#12844)

# Description

It's commonly forgotten or overlooked that a lot of `std repeat`
functionality can be handled with the built-in `fill`. Added 'repeat` as
a search term for `fill` to improve discoverability.

Also replaced one of the existing examples with one `fill`ing an empty
string, a la `repeat`. There were 6 examples already, and 3 of them
pretty much were variations on the same theme, so I repurposed one of
those rather than adding a 7th.

# User-Facing Changes

Changes to `help` only

# Tests + Formatting

- 🟢 `toolkit fmt`
- 🟢 `toolkit clippy`
- 🟢 `toolkit test`
- 🟢 `toolkit test stdlib`

# After Submitting

I assume the "Commands" doc is auto-generated from the `help`, but I'll
double-check that assumption.

crates/nu-command/src/conversions/fill.rs

@@ -73,7 +73,7 @@ impl Command for Fill {
     }
 
     fn search_terms(&self) -> Vec<&str> {
-        vec!["display", "render", "format", "pad", "align"]
+        vec!["display", "render", "format", "pad", "align", "repeat"]
     }
 
     fn examples(&self) -> Vec<Example> {
@@ -91,9 +91,9 @@ impl Command for Fill {
                 result: Some(Value::string("────────nushell", Span::test_data())),
             },
             Example {
-                description: "Fill a string on both sides to a width of 15 with the character '─'",
-                example: "'nushell' | fill --alignment m --character '─' --width 15",
-                result: Some(Value::string("────nushell────", Span::test_data())),
+                description: "Fill an empty string with 10 '─' characters",
+                example: "'' | fill --character '─' --width 10",
+                result: Some(Value::string("──────────", Span::test_data())),
             },
             Example {
                 description: