Document and extend index serialization format

[harsha-simhadri]

Is your feature request related to a problem? Please describe.

The current format for saving indices -- inmem and disk -- needs to be documented. It also does not allow for storing start point strategies.

Describe the solution you'd like

For both in-mem and SSD indices, extension of current format and documentation.

Steps

• Agree on requirements, target scenarios, exit criteria.
• Agree on proposed solution.
• Save index. Access artifacts.
• Backward compatibility