docs: `ActionForm` examples for indexing into struct fields (#2017) #1220

Job	Run time
get-leptos-changed / Detect Source Change	8s
CI (integrations/axum) / Run ci (nightly)	12m 11s
CI (integrations/viz) / Run ci (nightly)	12m 3s
CI (integrations/actix) / Run ci (nightly)	12m 17s
CI (integrations/utils) / Run ci (nightly)	12m 14s
CI (leptos) / Run ci (nightly)	46m 8s
CI (leptos_config) / Run ci (nightly)	11m 6s
CI (leptos_dom) / Run ci (nightly)	19m 12s
CI (leptos_hot_reload) / Run ci (nightly)	11m 46s
CI (leptos_macro) / Run ci (nightly)	17m 38s
CI (leptos_reactive) / Run ci (nightly)	26m 8s
CI (leptos_server) / Run ci (nightly)	18m 38s
CI (meta) / Run ci (nightly)	12m 6s
CI (router) / Run ci (nightly)	13m 28s
CI (server_fn) / Run ci (nightly)	12m 24s
CI (server_fn_macro) / Run ci (nightly)	11m 8s
CI (server_fn/server_fn_macro_default) / Run ci (nightly)	11m 47s
	4h 20m 22s

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