feat: add merge_all_wren utility, strutil demo, and 7 new API manual pages

Add WrenFileReader class in apps/merge_all_wren.wren for recursive .wren file discovery and content printing. Introduce example/strutil_demo.wren demonstrating case conversion, hex encoding, SHA256 hashing, string repetition, padding, HTML/JSON escaping, and URL encoding/decoding. Create manual/api pages for argparse, dataset, html, markdown, uuid, wdantic, and web modules. Update manual/api/index.html sidebar and module count from 21 to 28, adding links and cards for the new modules.

test/markdown/links_advanced.wren

@@ -0,0 +1,21 @@
+// retoor <retoor@molodetz.nl>
+
+import "markdown" for Markdown
+
+var multi = Markdown.toHtml("[First](http://a.com) and [Second](http://b.com)")
+System.print(multi.contains("<a href=\"http://a.com\">First</a>")) // expect: true
+System.print(multi.contains("<a href=\"http://b.com\">Second</a>")) // expect: true
+
+var withPath = Markdown.toHtml("[Docs](/docs/api/index.html)")
+System.print(withPath.contains("<a href=\"/docs/api/index.html\">Docs</a>")) // expect: true
+
+var imgAlt = Markdown.toHtml("![A cat](cat.jpg)")
+System.print(imgAlt.contains("<img src=\"cat.jpg\" alt=\"A cat\">")) // expect: true
+
+var imgUrl = Markdown.toHtml("![Logo](https://example.com/logo.png)")
+System.print(imgUrl.contains("<img src=\"https://example.com/logo.png\" alt=\"Logo\">")) // expect: true
+
+var textAround = Markdown.toHtml("Click [here](url) now")
+System.print(textAround.contains("Click ")) // expect: true
+System.print(textAround.contains("<a href=\"url\">here</a>")) // expect: true
+System.print(textAround.contains(" now")) // expect: true