Improve filter documentation (#3941)

* Organize Filter constants into dedicated lists

* Group validation filters somewhat

* Start describing some filters

* Move Validation filter documentation to constants.

* Move some sanitization filters

---------

Co-authored-by: Christoph M. Becker <[email protected]>

Author: Girgias

reference/filter/book.xml

@@ -18,7 +18,7 @@
    <emphasis>validation</emphasis> and <emphasis>sanitization</emphasis>.
   </para>
   <para>
-   <link linkend="filter.filters.validate">Validation</link> is used to
+   Validation is used to
    validate or check if the data meets certain qualifications. For example,
    passing in <constant>FILTER_VALIDATE_EMAIL</constant> will determine if
    the data is a valid email address, but will not change the data itself.