Adds more details to Atlas (Aeon/Ares/ILLiad) documentation

[acozine]

Follow up to #236.

• Moves the Tigris documentation into its own file.
• Creates a directory for Atlas-related documentation.
• Adds info on how the three Atlas applications are related.
• Moves the ILLiad documentation into a separate file, since it is not hosted.
• Adds info on how to get support for ILLiad.
• A few other refinements.

[acozine]

Rebased and updated to take account of changes from #274.

[kevinreiss]

Thanks @acozine.