Updated ssh.md and new 0_Index.md

[clstmbrly]

@lachellel @djpackham

• Updated ssh.md for Windows, Linux, and Mac (formerly called "ssh-all.md"): Removed "CAC" and made additional editorial changes. A Markdown solution for indented blocks resolved the rendering issue with out-of-sequence ordered lists. Some rendering issues still exist--too much space between some steps, as well as between Alert blocks and following text. Could not resolve the spacing issues.

• Converted Index.md to 0_Index.md and made a few editorial fixes to file.

Please let me know if any additional changes are needed.

[lachellel]

merging into the branch: ssh-all to iterate edits recommended:

• there is no navigation to help the reader find the information
• up front introduction information needed to explain this is using authorized keys and other options are available (pam modules tied to directories)
• less steps; no need to be explicit on the steps like explaining saved sessions, entering a hostname, etc
• removal of items around a jump server administrator / genericize
• introductory information on why this is done, using hardware vs software, etc