new docs.sourcegraph.com design

Created by: sqs

Goals:

• Improve/modernize look-and-feel
• Simplify sidebar content, implementation, and how-to-update process
• Add dark theme

Old sidebar

New sidebar

(Responsive and dark theme screenshots also shown)

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Relevant Slack discussions:

• Initial: https://sourcegraph.slack.com/archives/C07KZF47K/p1598388973084000?thread_ts=1598295302.057500&cid=C07KZF47K
• Recent: https://sourcegraph.slack.com/archives/C0HMGV90V/p1601510298050600