A11y: Heading hierarchy skips levels across multiple pages

## Description The heading hierarchy on several pages is incorrect. Heading levels are skipped (e.g., `<h1>` followed directly by `<h3>`, or `<h2>` followed by `<h6>`), which makes it difficult for screen reader users to understand the structure and relationships of the content. **Affected areas:** - **Profile page:** An `<h3>` heading appears directly after an `<h1>` heading, skipping `<h2>`. Additionally, the "Preferences" section has no heading at all. - **Home page:** An `<h6>` heading appears directly after `<h2>`, and `<h1>` is missing entirely. - Other pages have similar issues. ## Steps to reproduce 1. Open Grafana in Chrome on Windows 2. Enable JAWS screen reader 3. Navigate to the Profile page 4. Use the screen reader's heading navigation (e.g., press H in JAWS) to navigate through headings 5. Observe that the heading hierarchy skips from `<h1>` to `<h3>`, bypassing `<h2>` ## Actual behavior Heading levels are skipped across multiple pages, and some sections lack headings entirely. ## Expected behavior Headings should follow a logical, sequential order (e.g., `<h1>` followed by `<h2>`, then `<h3>`, etc.) to maintain a proper content hierarchy. All major sections should have appropriate headings. ## Suggested fix Audit heading structure across all pages and update them to follow a sequential hierarchy. For example, on the Profile page, replace the `<h3>` with `<h2>` if it is a direct subsection of the `<h1>`, and add an `<h2>` for the "Preferences" section. ## WCAG criteria - **1.3.1 Info and Relationships** (Level A) ## Possible duplicates in grafana/grafana - [#73693](https://github.com/grafana/grafana/issues/73693) — [Epic] Heading consistency across Grafana (incorrect hierarchy, additional headings required, incorrect headings) (open; tracks heading hierarchy issues across the application) - [#106841](https://github.com/grafana/grafana/issues/106841) — A11y: Audit H1 usage across Grafana (open; auditing heading usage) - [#66560](https://github.com/grafa…