docs (flashblocks): add overview, architecture, websocket-reference page, update content, improve readability/organization

[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 — transaction lifecycle, block building algorithm, and integration guidance
• New Architecture page — adds sequencer architecture and how flashblocks components operate
• New Websocket-reference page — Rename node-providers.mdx --> websocket-reference.mdx; convert from a guide to a reference page with tables and example json response bodies
• Adds dynamic mempool as Flashblocks block building difference vs. default OP stack block building
• Improved FAQ with accordion-style presentation
• Better formatting, tables, and cross-references
• Update Flashblocks content based on Flashblocks technical blog
• Updated docs.json — Added new pages to navigation

Notes to reviewers

How has it been tested?
Locally

[cb-heimdall]

🟡 Heimdall Review Status

Requirement	Status	More Info
Reviews	🟡 0/1	Denominator calculation Show calculation 1 if user is bot 0 1 if user is external 0 2 if repo is sensitive 0 From .codeflow.yml 1 Additional review requirements Show calculation Max 0 0 From CODEOWNERS 0 Global minimum 0 Max 1 1 1 if commit is unverified 0 Sum 1

Approved review 3712473321 from ericbrown99 is now dismissed due to new commit. Re-request for approval.