feat(blog): XSLT/XPath function reference section (Phase 2)

- 19 reference pages under /xslt/functions/ covering:
  xsl:value-of, xsl:for-each, xsl:for-each-group, xsl:function,
  xsl:iterate, last(), position(), count(), concat(), contains(),
  substring(), sum(), key(), current(), string-join(), tokenize(),
  matches(), replace(), format-date()
- Each page: description, parameters table, return value, 2 runnable
  examples (input XML + stylesheet + output), notes, cross-links
- New Hugo section with reference-specific layout (version badges,
  syntax block, "Try in Playground" CTA, parameters table styles)
- Menu entry "Reference" → /xslt/functions/
- CSS: .ref-badge, .ref-syntax, .ref-index-list, .ref-try + table styles

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

site/hugo.toml

@@ -35,6 +35,11 @@ publisher = "xsltplaygroundcom"
   url = "/about/"
   weight = 3
 
+  [[menu.main]]
+  name = "Reference"
+  url = "/xslt/functions/"
+  weight = 4
+
   [[menu.main]]
   name = "Charts"
   url = "https://alexandrev.github.io/xslt-lab/index.yaml"