docs (flashblocks): add overview page and improve structure

[roethke]

What changed? Why?
Improves Flashblocks documentation based on the Deep Dive blog post, which was proving more helpful for teams than the existing docs.

Updates:

• New Overview page — Architecture, transaction lifecycle, block building algorithm, and integration guidance
• Better formatting, tables, and cross-references
• Improved Flashblocks section readability
• Updated docs.json — Added overview to navigation

Notes to reviewers

How has it been tested?
Locally

[cb-heimdall]

🟡 Heimdall Review Status

Requirement	Status	More Info
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