add decommission cli command docs

[sascala]

Description

Urgency

• Blocker
• High
• Medium

Requirements

• Test all commands and procedures.
• Build content locally and test for formatting/links.
• Add redirects to dcos-website/redirect-files.
• Change all affected versions (e.g. 1.7, 1.8, 1.9, and 1.10).
• See the contribution guidelines.

[sascala]

@pavisandhu This needs to be merged into the new site