feat(docs): Update and apply kbd tag style guide (#2930)

- Based on feed back from @mgeisler, update the kbd tag guidance in
GEMINI.md to require wrapping each key in its own tag (e.g.,
`<kbd>Ctrl</kbd> + <kbd>S</kbd>`).
- Apply this new convention to existing content in the `src` directory
to ensure consistency.

src/cargo/code-samples.md

@@ -16,8 +16,8 @@ fn main() {
 }
 ```
 
-You can use <kbd>Ctrl + Enter</kbd> to execute the code when focus is in the
-text box.
+You can use <kbd>Ctrl</kbd> + <kbd>Enter</kbd> to execute the code when focus is
+in the text box.
 
 <details>