Update Docs UI/UX

Created by: francisschmaltz

Migrated from https://github.com/sourcegraph/docs.sourcegraph.com/pull/11

Update docs UI to be inline with our product UI

• Use main SG theme
• Update logo
• Update header
• Update text styles and colors
• Update layout
• Add mobile support
• Add docs content nav on left side

Fixes: https://github.com/sourcegraph/sourcegraph/issues/2464 Fixes: https://github.com/sourcegraph/sourcegraph/issues/2463 Fixes: https://github.com/sourcegraph/sourcegraph/issues/1701 Fixes: https://github.com/sourcegraph/sourcegraph/issues/2246 Fixes: https://github.com/sourcegraph/docs.sourcegraph.com/pull/10

Other features:

• Adds support for alternative search providers (Default: Google, DuckDuckGo, Sourcegraph.com)
• Adds clear footer
• Adds mobile support.

Desktop:

Mobile: