tpl/urls: Add JoinPath template function

See https://pkg.go.dev/net/url#JoinPath

Closes #9694

docs/content/en/functions/urls.JoinPath.md

@@ -0,0 +1,24 @@
+---
+title: urls.JoinPath
+description: Joins the provided elements into a URL string and cleans the result of any ./ or ../ elements.
+categories: [functions]
+menu:
+  docs:
+    parent: functions
+keywords: [urls,path,join]
+signature: ["urls.JoinPath ELEMENT..."]
+---
+
+```go-html-template
+{{ urls.JoinPath "" }} → "/"
+{{ urls.JoinPath "a" }} → "a"
+{{ urls.JoinPath "a" "b" }} → "a/b"
+{{ urls.JoinPath "/a" "b" }} → "/a/b"
+{{ urls.JoinPath "https://example.org" "b" }} → "https://example.org/b"
+
+{{ urls.JoinPath (slice "a" "b") }} → "a/b"
+```
+
+Unlike the [`path.Join`] function, `urls.JoinPath` retains consecutive leading slashes.
+
+[`path.Join`]: /functions/path.join/