docs: improve resources.md with spec link and concrete-URI examples

Added a link to the MCP resources specification after the intro.

Rewrote the multi-segment paths section to lead with the problem:
show a URI that fails with {name} before introducing {+name} as the
fix. Code comments align inputs with outputs for at-a-glance parsing.

Rewrote the query parameters section to lead with the two concrete
URIs a user would want to support (base and with-query), then show
how one template covers both.