feat: Add g17 event registration form

Step 1: What is changing in this MR?

• Add /seventeen registration form with content under /content module

Production	Review app
https://about.gitlab.com/	https://feat-git-17-reg.about.gitlab-review.app/seventeen/

Non-exhaustive list of pages using form:

Production	Review app
https://about.gitlab.com/fifteen/	https://feat-git-17-reg.about.gitlab-review.app/fifteen/
https://about.gitlab.com/sixteen/	https://feat-git-17-reg.about.gitlab-review.app/sixteen/
https://about.gitlab.com/solutions/gitlab-duo-pro/sales/	https://feat-git-17-reg.about.gitlab-review.app/solutions/gitlab-duo-pro/sales/
https://about.gitlab.com/sales/	https://feat-git-17-reg.about.gitlab-review.app/sales/
https://about.gitlab.com/services	https://feat-git-17-reg.about.gitlab-review.app/services/
https://about.gitlab.com/company/contact/	https://feat-git-17-reg.about.gitlab-review.app/company/contact/
https://about.gitlab.com/value-stream-workshop/	https://feat-git-17-reg.about.gitlab-review.app/value-stream-workshop/

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

Related to https://gitlab.com/gitlab-com/marketing/digital-experience/buyer-experience/-/issues/3687