SEO the cloud site only

Created by: aileenrose

Feature request description

Improve SEO to home page and repositories group pages. Similar request logged for docsite pages: https://github.com/sourcegraph/docsite/issues/65

Additional context

updated Dec 7, 2020

1. Allow the ability to add meta description for each repository group page i.e. https://sourcegraph.com/refactor-python2-to-3

Currently, the default is the following for all pages: Sourcegraph is a web-based code search and navigation tool for dev teams. Search, navigate, and review code. Find answers.

2. Create a sitemap of the primary pages

• include pages of repo such as: -- https://sourcegraph.com/react-hooks, -- https://sourcegraph.com/refactor-python2-to-3
• Exclude: https://sourcegraph.com/search?q=