Create Slack integration marketing landing page

This MR creates a landing page for the GitLab Slack integration at /solutions/slack/.

Review App: https://1281-eng-slack-integration-marketing-landing-page.about.gitlab-review.app/solutions/slack/

• - [ ] I, the Assignee, have run Axe Devtools on any updated pages, and fixed the relevant accessibility issues.
• These changes meet a specific OKR or item in our Quarterly Plan.
• These changes work on both Safari and Chrome.
• 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.
• Images are appropriately sized

Closes #1281 (closed)