feat: Resolve "Features Page - Improve Anchor Link Discoverability"

Step 1: What is changing in this MR?

• Clicking on anchor links now updates the address bar with the respective link route
• Adds anchor icons to feature headers for easy discovery when navigating the page

Production	Review app
https://about.gitlab.com/features	https://3647-features-page-improve-anchor-link-discoverability.about.gitlab-review.app/features/

Step 2: Ensure that your changes comply with the following, where applicable:

• I, the Assignee, have run Axe tools on any updated pages, and fixed the relevant accessibility issues.
• These changes work on both Safari, Chrome, and Firefox.
• These changes have been reviewed for Visual Quality Assurance and Functional Quality Assurance on Mobile, Desktop, and Tablet.
• These changes work with our Google Analytics and SEO tools.
• These changes have been documented as expected.

Build Variables:

• Use Contentful Preview API

Closes #3647 (closed)