docs: use gfm notes in README (#952)

A fairly new addition to GitHub Flavored Markdown.

---------

Signed-off-by: Henry Schreiner <[email protected]>
Co-authored-by: pre-commit-ci[bot] <66853113+pre-commit-ci[bot]@users.noreply.github.com>
Co-authored-by: Philip Top <[email protected]>

README.md

@@ -1025,9 +1025,11 @@ option_groups. These are:
   command line for a flag. The operation will throw an exception if the option
   name is not valid.
 
-> Note: if you have a fixed number of required positional options, that will
-> match before subcommand names. `{}` is an empty filter function, and any
-> positional argument will match before repeated subcommand names.
+> [!NOTE]
+>
+> If you have a fixed number of required positional options, that will match
+> before subcommand names. `{}` is an empty filter function, and any positional
+> argument will match before repeated subcommand names.
 
 #### Callbacks
 
@@ -1411,68 +1413,70 @@ function may already receive broken Unicode. Parsing `argv` at that point will
 not give you a correct string. To fix this, you have three good options and two
 bad ones:
 
-1. Replace `argv` with `app.ensure_utf8(argv)` before any arguments are parsed.
-   `ensure_utf8` will do nothing on systems where `argv` is already in UTF-8
-   (Such as Linux or macOS) and return `argv` unmodified. On Windows, it will
-   discard `argv` and replace it with a correctly decoded array or arguments
-   from win32 API.
-
-   ```cpp
-   int main(int argc, char** argv) {
-       CLI::App app;
-       argv = app.ensure_utf8(argv);  // new argv memory is held by app
-       // ...
-       CLI11_PARSE(app, argc, argv);
-   }
-   ```
-
-2. If you pass unmodified command-line arguments to CLI11, call `app.parse()`
-   instead of `app.parse(argc, argv)` (or `CLI11_PARSE(app)` instead of
-   `CLI11_PARSE(app, argc, argv)`). The library will find correct arguments by
-   itself.
-
-   Note: this approach may not work on weird OS configurations, such as when the
-   `/proc` dir is missing on Linux systems (see also
-   [#845](https://github.com/CLIUtils/CLI11/issues/845)).
-
-   ```cpp
-   int main() {
-       CLI::App app;
-       // ...
-       CLI11_PARSE(app);
-   }
-   ```
-
-3. Get correct arguments with which the program was originally executed using
-   provided functions: `CLI::argc()` and `CLI::argv()`. These three methods are
-   the only cross-platform ways of handling unicode correctly.
-
-   ```cpp
-   int main() {
-       CLI::App app;
-       // ...
-       CLI11_PARSE(app, CLI::argc(), CLI::argv());
-   }
-   ```
+1\. Replace `argv` with `app.ensure_utf8(argv)` before any arguments are parsed.
+`ensure_utf8` will do nothing on systems where `argv` is already in UTF-8 (Such
+as Linux or macOS) and return `argv` unmodified. On Windows, it will discard
+`argv` and replace it with a correctly decoded array or arguments from win32
+API.
+
+```cpp
+int main(int argc, char** argv) {
+    CLI::App app;
+    argv = app.ensure_utf8(argv);  // new argv memory is held by app
+    // ...
+    CLI11_PARSE(app, argc, argv);
+}
+```
+
+2\. If you pass unmodified command-line arguments to CLI11, call `app.parse()`
+instead of `app.parse(argc, argv)` (or `CLI11_PARSE(app)` instead of
+`CLI11_PARSE(app, argc, argv)`). The library will find correct arguments by
+itself.
+
+> [!NOTE]
+>
+> This approach may not work on weird OS configurations, such as when the
+> `/proc` dir is missing on Linux systems (see also
+> [#845](https://github.com/CLIUtils/CLI11/issues/845)).
+>
+> ```cpp
+> int main() {
+>     CLI::App app;
+>     // ...
+>     CLI11_PARSE(app);
+> }
+> ```
+
+3\. Get correct arguments with which the program was originally executed using
+provided functions: `CLI::argc()` and `CLI::argv()`. These three methods are the
+only cross-platform ways of handling unicode correctly.
+
+```cpp
+int main() {
+    CLI::App app;
+    // ...
+    CLI11_PARSE(app, CLI::argc(), CLI::argv());
+}
+```
 
 <details><summary>Bad options (click to expand)</summary><p>
 
-4. Use the Windows-only non-standard `wmain` function, which accepts
-   `wchar_t *argv[]` instead of `char* argv[]`. Parsing this will allow CLI to
-   convert wide strings to UTF-8 without losing information.
-
-   ```cpp
-   int wmain(int argc, wchar_t *argv[]) {
-       CLI::App app;
-       // ...
-       CLI11_PARSE(app, argc, argv);
-   }
-   ```
-
-5. Retrieve arguments yourself by using Windows APIs like
-   [`CommandLineToArgvW`](https://learn.microsoft.com/en-us/windows/win32/api/shellapi/nf-shellapi-commandlinetoargvw)
-   and pass them to CLI. This is what the library is doing under the hood in
-   `CLI::argv()`.
+4\. Use the Windows-only non-standard `wmain` function, which accepts
+`wchar_t *argv[]` instead of `char* argv[]`. Parsing this will allow CLI to
+convert wide strings to UTF-8 without losing information.
+
+```cpp
+int wmain(int argc, wchar_t *argv[]) {
+    CLI::App app;
+    // ...
+    CLI11_PARSE(app, argc, argv);
+}
+```
+
+5\. Retrieve arguments yourself by using Windows APIs like
+[`CommandLineToArgvW`](https://learn.microsoft.com/en-us/windows/win32/api/shellapi/nf-shellapi-commandlinetoargvw)
+and pass them to CLI. This is what the library is doing under the hood in
+`CLI::argv()`.
 
 </p></details>
 </br>