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/paragraphs.wren

@@ -0,0 +1,17 @@
+// retoor <retoor@molodetz.nl>
+
+import "markdown" for Markdown
+
+var p = Markdown.toHtml("This is a paragraph.")
+System.print(p) // expect: <p>This is a paragraph.</p>
+
+var multi = Markdown.toHtml("First paragraph.\n\nSecond paragraph.")
+System.print(multi.contains("<p>First paragraph.</p>")) // expect: true
+System.print(multi.contains("<p>Second paragraph.</p>")) // expect: true
+
+var afterHeading = Markdown.toHtml("# Title\n\nSome text here.")
+System.print(afterHeading.contains("<h1>Title</h1>")) // expect: true
+System.print(afterHeading.contains("<p>Some text here.</p>")) // expect: true
+
+var empty = Markdown.toHtml("")
+System.print(empty) // expect: