Account creation for Restricted Accounts

[seaxwi]

This PR fixes the following:

Issue 1: Age limit for parental consent requirement

Create an Arduino account for a minor states:

Minors under the age of 14 need parental consent to sign up for an Arduino account.

This is misleading, since parental consent is required for any minor under the age of 18.

Issue 2 Missing instructions for Restricted Accounts

Create an Arduino account for a minor only contains instructions for creating a Junior Account.

The article title suggests that any minor account should be covered. The way this article is linked from About Arduino accounts for minors also expects the article to address this:

For a minor to create a Restricted Account...

Instructions for both account types should be included.

Issue 3: Incorrect description of sign-up flow

About Arduino accounts for minors correctly states:

When a Junior Account is upgraded to a Restricted Account, the user will be prompted to provide a username of their own choosing and an email address that will be linked to their account.

But also:

For a minor to create a Restricted Account, the user is asked to choose a username and a picture (an Avatar) from a set of predetermined options, to enter a password of their choice, and to provide their date of birth.

This description only applies to junior accounts. This should be updated to be consistent with the first paragraph.