docs: add URI extension reference to manual (#273)

Add reference documentation for the URI extension to the PHP manual, listing it alphabetically between tokenizer and url extensions.

Author: jordikroon

manual.xml

@@ -301,6 +301,7 @@
    &reference.swoole.book;
    &reference.tidy.book;
    &reference.tokenizer.book;
+   &reference.uri.book;
    &reference.url.book;
    &reference.v8js.book;
    &reference.yaml.book;