(#21970) Comment fixup

The REST certificate endpoint supports only find, not save.
(#21970) Move supported formats into the per-endpoint md
(#21970) Add markdown doc for certificate endpoint

Add markdown doc and add link from index markdown page.
(#21970) Correct markdown and add explanatory text
(#21970) note that the certificate is in pem format
(#21970) add 'Supported HTTP Methods' section
Merge pull request #1823 from kylog/issue/21970-document-certificate

(#21970) document certificate endpoint