[Doc] Document zInfo binary format and implementation details

[austinvazquez]

Description

There is growing interest by the community to build independent SOCI index generators. For example, #1117 documents interest in an independent rust crate and also references ztoc-rs for doing this.

For these users, expanding on the zInfo binary format and implementation details would be helpful to understand the format and contents of the SOCI artifacts.

Describe the solution you'd like

Expand docs/index.md to explicitly document the FlatBuffers format and zInfo implementation details.

Describe any alternative solutions/features you've considered

No response

Any additional context or information about the feature request

This is follow-up to #757

[aochagavia]

Great! An additional thing to mention, if anyone picks this up, is in how far the zInfo is tied to a particular compression method (e.g. would it need to be modified if / when SOCI supports zstd?)