docs: refactor Lance format specification structure

[jackye1995]

Summary

• refactor the Lance format specification so index formats are a top-level section
• split catalog specs and namespace client spec into distinct sections in the published docs
• move full website assembly into reusable docs scripts and CI wiring
• keep the docs buildable with placeholders when external repos are absent

Namespace PR: lance-format/lance-namespace#326
Vote: #6567
Preview: https://jackye1995.github.io/lance/