style: align stats/base/dists/gamma/logcdf with namespace conventions

Restructured the README to match the structural majority pattern used
across the `stats/base/dists/gamma` namespace:

- Wrapped the C APIs section in `<section class="c">` (78.6% of siblings).
- Promoted `### C APIs` heading to `## C APIs` (78.6% of siblings).
- Promoted `#### Usage` heading under C APIs to `### Usage` (78.6% of siblings).
- Closed `<section class="usage">` for the JS API before the C APIs section
  rather than after it; the C APIs content is now a sibling, not nested
  inside the JS usage wrapper.

No content changes; section layout and heading levels only.

Author: claude

lib/node_modules/@stdlib/stats/base/dists/gamma/logcdf/README.md

@@ -125,9 +125,17 @@ y = mylogcdf( 4.0 );
 // returns ~-0.064
 ```
 
+</section>
+
+<!-- /.usage -->
+
+<!-- C interface documentation. -->
+
 * * *
 
-### C APIs
+<section class="c">
+
+## C APIs
 
 <!-- Section to include introductory text. Make sure to keep an empty line after the intro `section` element and another before the `/section` close. -->
 
@@ -141,7 +149,7 @@ y = mylogcdf( 4.0 );
 
 <section class="usage">
 
-#### Usage
+### Usage
 
 ```c
 #include "stdlib/stats/base/dists/gamma/logcdf.h"
@@ -220,7 +228,7 @@ int main( void ) {
 
 </section>
 
-<!-- /.usage -->
+<!-- /.c -->
 
 <section class="examples">