Additional changes to code-xy pages to improve readability

Created by: aileenrose

I updated code pages to improve readability for new visitors to the pages.
On the code review page, I included an excerpt of the GitLab video which focuses on code discovery and code review. https://deploy-preview-1055--sourcegraph.netlify.app/product/code-review https://deploy-preview-1055--sourcegraph.netlify.app/product/code-discovery https://deploy-preview-1055--sourcegraph.netlify.app/product/code-intelligence

Note: I did not remove the CTA demo request from the code change mgmt page since this feature is in beta.