Remove built-in help docs, link to the docs.sourcegraph.com/@version instead

Created by: ryan-blunden

There are several issues (#1885 (closed), #1720 (closed), #1912 (closed)) caused by styling or functionality differences between the in-product help and docs.sourcegraph.com

I propose removing the in-product help and instead, link to the version of the docs the instance is running.

This has the following benefits:

• Still provides version specific docs.
• No styling and functionality consistency issues so we can close #1885 (closed), #1720 (closed) and #1912 (closed)
• No light and dark theme styles for help docs.
• Removes Sourcegraph's dependency on docsite.

In summary removing the in-product help still provides users with version specific docs while saving effort now, and in the future.