docs: document experimental templ packages (#868)

Co-authored-by: Adrian Hesketh <adrianhesketh@hushmail.com>

docs/docs/13-experimental/01-overview.md

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+# Experimental packages
+
+Experimental Packages for templ are available at https://github.com/templ-go/x/
+
+:::warning
+- Packages in this module are experimental and may be removed at any time.
+- There is no guarantee of compatibility with future versions.
+- There is no guarantee of stability.
+- Use at your own risk.
+:::
+
+## Approval Process
+
+As of right now, there is no formal approval process for packages to be stabilized and moved into https://github.com/a-h/templ. Feel free to contribute via GitHub discussions at https://github.com/a-h/templ/discussions

docs/docs/13-experimental/02-urlbuilder.md

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+# urlbuilder
+
+A simple URL builder to construct a `templ.SafeURL`.
+
+```templ title="component.templ"
+import (
+  "github.com/templ-go/x/urlbuilder"
+  "strconv"
+  "strings"
+)
+
+templ component(o Order) {
+  <a
+    href={ urlbuilder.New("https", "example.com").
+    Path("orders").
+    Path(o.ID).
+    Path("line-items").
+    Query("page", strconv.Itoa(1)).
+    Query("limit", strconv.Itoa(10)).
+    Build() }
+  >
+    { strings.ToUpper(o.Name) }
+  </a>
+}
+```
+
+See [URL Attribures](/syntax-and-usage/attributes#url-attributes) for more information.
+
+## Feedback
+
+Please leave your feedback on this feature at https://github.com/a-h/templ/discussions/867

docs/docs/13-experimental/_category_.json

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+{
+  "position": 13,
+  "label": "Experimental"
+}

docs/docs/13-help-and-community/_category_.json → docs/docs/14-help-and-community/_category_.json

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 {
-  "position": 13,
+  "position": 14,
   "label": "Help and community"
 }

docs/docs/14-faq/_category_.json → docs/docs/15-faq/_category_.json

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 {
-  "position": 14,
+  "position": 15,
   "label": "FAQ"
 }