Separate digest auth from HTTP headers parsing

PUBLISHED_FROM=f97569ae2f31ab94ce4875eae4d0a198f719c388

docs/c-api/http_server.h/intro.md

@@ -3,6 +3,7 @@ title: "Server API reference"
 symbol_kind: "intro"
 decl_name: "http_server.h"
 items:
+  - { name: mg_check_digest_auth.md }
   - { name: mg_file_upload_handler.md }
   - { name: mg_get_http_basic_auth.md }
   - { name: mg_get_http_header.md }