Fix #107: Validate <meta> data

Implemented as Schematron schema

geekodoc/sch/2_5.2/geekodoc-v2.sch

@@ -0,0 +1,39 @@
+<?xml version="1.0" encoding="UTF-8"?>
+<!--
+   GeekoDoc Schematron schema v2
+
+Purpose:
+   Validate elements, attributes, or other structures that are difficult
+   to validate in the GeekoDoc RNG schema.
+
+Author:  Thomas Schraitle, 2023
+-->
+<s:schema xmlns:s="http://purl.oclc.org/dsdl/schematron"
+  xmlns:db="http://docbook.org/ns/docbook"
+  xmlns:pr="http://www.schematron-quickfix.com/validator/process"
+   queryBinding="xslt"
+>
+  <s:ns prefix="db" uri="http://docbook.org/ns/docbook"/>
+  <s:ns prefix="xlink" uri="http://www.w3.org/1999/xlink"/>
+  <s:ns prefix="trans" uri="http://docbook.org/ns/transclusion"/>
+
+  <!-- Variables -->
+  <s:let name="meta-title-length" value="55"/>
+  <s:let name="meta-description-length" value="150"/>
+
+  <s:pattern>
+    <s:title>Validate meta elements</s:title>
+    <s:rule context="db:meta[@name='title']">
+      <s:assert test="string-length(.) &lt;= $meta-title-length">
+        The element meta[@name="title"] cannot have more than <s:value-of select="$meta-title-length"/> characters.
+      </s:assert>
+    </s:rule>
+
+    <s:rule context="db:meta[@name='description']">
+      <s:assert test="string-length(.) &lt;= $meta-description-length">
+        The element meta[@name="description"] cannot have more than <s:value-of select="$meta-description-length"/> characters.
+      </s:assert>
+    </s:rule>
+  </s:pattern>
+
+</s:schema>