Fix heading-order accessibility: configurable heading level on the Share component

Problem/Motivation

An axe-core accessibility audit of the Educare site template found a moderate WCAG 2.4.6 (Headings and Labels) heading-order violation repeated across the template: heading levels skip (e.g. a section <h2> followed directly by a card <h5>, or an <h1>→<h2>→<h5> gap). It was found on the homepage, /programs, a program page, a news article, /events, an event page and /about.

On the theme side, one root cause is the Share (webshare) molecule: its title always renders as a hardcoded <h3>, regardless of the heading level the surrounding section actually needs, so it breaks the outline whenever it sits under anything other than an <h2> section. See also the related recipe issue #3614489, which fixes the same audit finding on the Educare recipe's cards and FAQ accordion.

Steps to reproduce

1. Install the Educare site template, or open any Educare page that renders a Share block (e.g. a news article or an event page).
2. Run an axe-core heading-order check on that page (e.g. the Varbase functional testing suite's a11y step, or axe DevTools).
3. Observe the heading-order violation: the Share block's <h3> title does not follow the heading level of the section it is placed in.

Proposed resolution

Decouple the heading LEVEL from the visual SIZE:

• Add a heading_level prop to components/molecules/share/share.component.yml (integer, enum 2-6, default 2).
• Use it in share.twig: <h{{ heading_level }}> instead of the hardcoded <h3>.
• Visual size stays fixed via the existing webshare__heading class, so callers can set the correct semantic level without changing how the title looks.

Verified: axe heading-order is 0 across the 7 affected pages on a fresh install; card/heading visual sizes are unchanged; gitlab-ci-local is green on this repo. Merged as MR !22.

Remaining tasks

• ✅ File an issue about this project
• ✅ Addition/Change/Update/Fix to this project
• ✅ Testing to ensure no regression
• ➖ Automated unit/functional testing coverage
• ➖ Developer Documentation support on feature change/addition
• ➖ User Guide Documentation support on feature change/addition
• ➖ UX/UI designer responsibilities
• ✅ Accessibility and Readability
• ❌ Reviewed by a human
• ❌ Code review by maintainers
• ✅ Full testing and approval
• ✅ Credit contributors
• ✅ Review with the product owner
• ✅ Update Release Notes
• ✅ Release

User interface changes

• N/A

API changes

• N/A

Data model changes

• N/A

Release notes snippet

• Fixed a heading-order accessibility issue on the Share component by making its heading level configurable.