Wiki/Requirements: add links to the docs for each PHP extension

Author: jrfnl

wiki/Requirements.md

@@ -1,19 +1,26 @@
-PHP_CodeSniffer 4.x requires PHP 7.2.0 or greater.  
+PHP_CodeSniffer 4.x requires PHP 7.2.0 or greater.
 _Prior to PHP_CodeSniffer 4.0.0, the minimum PHP requirement was 5.4.0._
 
 Additionally, PHP_CodeSniffer requires the following PHP extensions to be enabled:
-- Tokenizer: used by the core tokenizer to process PHP files
-- SimpleXML: used to process ruleset XML files
-- XMLWriter: used to create some report formats
+- [Tokenizer][tokenizer]: used by the core tokenizer to process PHP files
+- [SimpleXML][simplexml]: used to process ruleset XML files
+- [XMLWriter][xmlwriter]: used to create some report formats
 
 The following PHP extensions are not required, but are strongly recommended:
-- iconv: used for accurate character length calculation in files containing multibyte characters. Without this extension, some sniffs, like `Generic.Files.LineLength`, may report incorrect results for lines containing non-ASCII characters, as PHP_CodeSniffer will fall back to byte-based length calculations.
-- PCNTL: required for parallel processing via the `--parallel` CLI option. Without this extension, PHP_CodeSniffer will not be able to check multiple files simultaneously.
+- [iconv]: used for accurate character length calculation in files containing multibyte characters. Without this extension, some sniffs, like `Generic.Files.LineLength`, may report incorrect results for lines containing non-ASCII characters, as PHP_CodeSniffer will fall back to byte-based length calculations.
+- [PCNTL][pcntl]: required for parallel processing via the `--parallel` CLI option. Without this extension, PHP_CodeSniffer will not be able to check multiple files simultaneously.
 
 Individual sniffs may have additional requirements such as external applications and scripts. See the [Configuration Options](https://github.com/PHPCSStandards/PHP_CodeSniffer/wiki/Configuration-Options) manual page for a list of these requirements.
 
 > [!WARNING]
-> The gRPC PHP extension is known to cause PHP_CodeSniffer to hang when running with parallel processing enabled. If the gRPC extension is loaded, either do not use the `--parallel` CLI option or configure the extension's ini settings as follows:
+> The [gRPC PHP extension][grpc] is known to cause PHP_CodeSniffer to hang when running with parallel processing enabled. If the gRPC extension is loaded, either do not use the `--parallel` CLI option or configure the extension's ini settings as follows:
 > ```text
 > phpcs -d grpc.enable_fork_support=1 -d grpc.poll_strategy=epoll1 [other options] <file|directory>
 > ```
+
+[grpc]:      https://grpc.io/docs/languages/php/
+[iconv]:     https://www.php.net/book.iconv
+[pcntl]:     https://www.php.net/book.pcntl
+[simplexml]: https://www.php.net/book.simplexml
+[tokenizer]: https://www.php.net/book.tokenizer
+[xmlwriter]: https://www.php.net/book.xmlwriter