docs: worked component-authoring examples in components guide

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What

Fills a gap in guides/components.md: the guide documented every built-in component well but only had a thin fragment for authoring your own components and dropping them into a screen.

Replaces the old "Pure-Elixir composite components" section with a "Defining your own components" section containing two complete, runnable screen modules:

• Function composite (MyApp.DashboardScreen): defines stat_card/3, uses it via {...} interpolation, and wires a <Button> inside the composite back to the screen's handle_info/2.
• Tag composite (<Card> + <LabeledButton>): the two expander modules (modeled on MobDemoKit.Card), both registration paths (manifest ui_components and runtime Mob.Composite.register/2), and a consuming MyApp.ProfileScreen.

Why it matters

Two things that were previously implicit and easy to get wrong are now explicit:

• The tag-to-atom rule (<Card> -> :card, verified against Mob.Sigil's Macro.underscore), so registration atoms line up.
• Where on_* auto-injection applies: only a composite tag's own props are shaped to {screen_pid, tag}. A plain <Button>/<TextField> placed directly (even inside a composite's children) still needs an explicit {self(), tag}. The two examples deliberately contrast these.

Scope

Docs only, single file (guides/components.md). No code changes. Section heading renamed from "Pure-Elixir composite components" to "Defining your own components"; confirmed nothing links to the old anchor.

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