std/url: URLs read, written, resolved and percent-encoded

The standard library chapter gets std/url after std/http: the quoted
literal that encodes each value for its part, parse-url's strict reading,
the round trip, url-resolve, the decoded parts and the encoders.
modules/std/url.sz is the reference page, generated from (std url)'s docs.

🤖 Generated with [ECA](https://eca.dev) (anthropic/claude-opus-4-7)

Co-Authored-By: eca-agent <git@eca.dev>

modules/std/url.sz

@@ -0,0 +1,3 @@
+(meta (title "(std url)"))
+
+@generate-docs[#:module "std url"]