* Adding new credential management docs

* Changed titles in TOC to match doc titles

* More clean-up of about and retrieve pages.

* Cleaned up store cred doc

* Copy edit about-credential-management-api.md.

* Copy edit retrieve-credentials.md.

* Copy edit store-credentials.md.

* Fix caption formatting in about-credential-management-api.md.

* Pete's edits

* code wrap

* edited with Eiji