doc: add helm docs

Created by: michaellzc

DO NOT MERGE until helm GA

Part of https://github.com/sourcegraph/sourcegraph/issues/32180

Test plan

sg run docsite   

Check below pages

• http://localhost:5080/admin/install/kubernetes
• http://localhost:5080/admin/install/kubernetes/configure#getting-started
• http://localhost:5080/admin/install/kubernetes/helm