Aggregator docs need an upgrade

[mhasself]

Improvements coming in #137 ... but some organizational / accuracy issues should also be resolved:

• The "Aggregator Agent" has a sub-section called "Aggregator Agent"...
• Perhaps present the "standard" Agent description blocks (site config, docker... ) first, then add details about the file structure.
• Refer to "spt3g_software" and "G3 Frames"
• SO3G is not a modification of SPT3G file format. It's a schema that operates on (now) standard G3 frames. And better to reserve "so3g" for the library. Our HK format is called the "SO HK Format".
• Reading the HK files is best done with methods from so3g.hk. Refer.