WIP: Reorganize Base Chain docs IA

[ericbrown99]

Summary

• Reorganizes Base Chain docs around current builder/operator jobs and adds a new Chain overview.
• Consolidates Flashblocks content into canonical Chain surfaces for block building, finality, RPC behavior, and node operation.
• Updates Azul activation, current network limits, endpoint/explorer references, redirects, and LLM indexes for the new IA.

Rationale

This keeps the recent Chain docs work coherent as a single reviewable change: navigation, redirects, factual network references, API behavior, and AI-facing indexes all move together instead of landing in a partially inconsistent state.

Validation

• Rebased onto latest origin/master.
• Ran stale literal/link QA sweep for the Chain docs edits.
• Ran node scripts/lint-mdx.js; it fails repo-wide on existing unrelated issues. The only PR-adjacent lint item is docs/snippets/base-networks.mdx missing frontmatter/no heading, matching existing imported-snippet patterns.

WIP

Opened as draft and labeled do not merge because the repo has no WIP label.

[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

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