Add Formatting Full Messages with I18n section in ActiveRecordValidations guides [ci-skip]

[AlexB52]

Summary

This PR adds a section on the ActiveRecordValidation guides to demonstrate how full validation messages can be formatted with the help of I18n.

Other Information

This PR was extracted from the PR proposal: #42708
While the feature didn't get any traction, the documentation of the existing behaviour to format full messages is currently missing.

[jonathanhefner]

What do you think about putting this information under config.active_model.i18n_customize_full_message instead? We could perhaps streamline the examples to use a single model with ActiveModel::Validations and a single validation on two different attributes. Then we could add a comment like # or `activerecord:` if using Active Record validations next to the activemodel: key in the YAML.

[AlexB52]

@jonathanhefner the suggested change sounds good.

I really feel that it's relevant for validations and i18n too and left in the configuration page is really hiding the information. Can we also add a link on these guides to the configuration example?

I'm thinking here: 7.3 errors.where and error object

under

The full_message method generates a more user-friendly message, with the capitalized attribute name prepended.

and here: active-model-methods

on this paragraph

ActiveModel::Errors#full_messages prepends the attribute name to the error message using a separator that will be looked up from errors.format (and which defaults to "%{attribute} %{message}").

I'll put some links in the meantime.
Cheers

[jonathanhefner]

I really feel that it's relevant for validations and i18n too. Can we also add a link on these guides to the configuration example?

@AlexB52 Sounds good! 👍

[jonathanhefner]

Just a few minor comments. Almost there! 😃

Also, when you're finished, would you mind squashing the commits?

[jonathanhefner]

Let's move this reference below the example, at the end of this section, above the ### Configuring Active Record heading.

[AlexB52]

Here I just moved the link below the code example. I hope this is what you meant and not the paragraph starting with
when set to 'true'...

[AlexB52]

@jonathanhefner
I removed the intermediary commit messages during the squash as I thought that wasn't really helpful and would be more noise. I left the co-authoring statement.

Can you please check if the commit message suits you?

I have a backup of all the commits in case a standard squash with the 12 commits in the description is necessary.

Thanks for the help
Cheers

[jonathanhefner]

There was a commit that accidentally removed a line break. (I probably should have included the line break in #45470 (comment).) I forced pushed a commit to fix it though, so we're good to go. Thank you for working on this, @AlexB52! 👍

(Backported to 7-0-stable.)