Headings and forms accessibility improvements

[shyusu4]

This PR addresses accessibility issues related to heading levels and forms:

• Fixed instances (in hero, headline, and icon-link blocks) where headings were used consecutively after one another.
• Changed menu_get_started to use an h2

• 457

• 459

• Currently screen reader announces sign up forms on the site with "Enter your email address" repeated 3 times. Fixed by changing base_form to use aria-label="Email" instead of <label>
• Changes the use of fieldset in search-form to a div (fieldset is used to group multiple form fields together, but there's only one here)

Screenshots

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[Scotchester]

Fixed instances (in hero, headline, and icon-link blocks) where headings were used consecutively after one another.

I looked for an issue in the audit spreadsheet that this addresses but couldn't easily find one. Could you point it out for me?

I'm a bit concerned about the addition of hgroup elements. They're probably harmless in the short term, but according to MDN, they have been removed from the HTML spec and are discouraged from being used. If a wrapper is helpful for applying the role="group" aria-roledescription="heading group" attributes, a div is probably best in this case.