[RF-DOCS] Action View Form Helpers Guide [ci-skip] (rails#51936)

In addition to updating the code examples and editing text for clarity and flow, here some more details on some of the changes:

- [X] Do we still need an HTML5 warning at the end of the 1st section? It's pretty standard these days.
--> agree, removed.
- [X] Not sure the output of `_form_with model: @article`_ is 100% correct, might need to double check that.
--> yes. updated this example and got the HTML output from my "guides playground" rails app.
- [X] When we mention `record.persisted?` in record identification, could be a good plug to link to the Active Model guide on that.
--> Hm...there is no direction mention, other than this https://guides.rubyonrails.org/active_model_basics.html#conversion
- [X] Similar to STI, link to the guide / reference on it.
- [X] Time Zone and Country Select should likely be broken into separate sub-sections (I don't mind still mentioning country select)
- [X]  Would the file upload example would be better with a CSV for local processing, rather than showing saving to local disk? (which is probably a very uncommon usage?)
- [X] The _labeled_form_with_ example could likely be simplified with `_**options_` being all that it takes, instead of explicitly showing all possible kwargs.
- [X]  It may be better to show "complex forms" (section 10) right after the parameters (section 8), moving "forms to external resources" down... just because complex forms require exactly the fields_for incantation that was detailed further under the parameters section, so it seems a better continuation. (or potentially even reversed? complex forms then params? not sure)
- [X] Under "complex forms", adding fields on the flow could be slightly expanded, it feels very "go figure". 
--> Yeah, not sure what to do about this one. Was thinking about removing it as there is not built-in support to showcase.

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Co-authored-by: Ridhwana <[email protected]>
Co-authored-by: Petrik de Heus <[email protected]>
Co-authored-by: Amanda Perino <[email protected]>
Co-authored-by: Karl Lingiah <[email protected]>

Author: 5 people