docs: add dedicated api reference pages for http, wss

[roethke]

What changed? Why?
Add JSON-RPC and Flashblocks API reference docs for Base chain

• New page: base-chain/reference/json-rpc-api.mdx — Comprehensive single-page JSON-RPC API reference covering all 30 methods supported by Base nodes (HTTP and WebSocket), with full parameter types, return value schemas, request/response examples, and error codes for every method.
• New page: base-chain/flashblocks/api-reference.mdx — Dedicated Flashblocks API reference documenting standard methods with pending tag behavior, Flashblocks-specific methods, and the three specialized WebSocket subscription types.
• Updated: base-chain/flashblocks/apps.mdx — Removed the inline RPC method docs, replaced with a concise endpoint table and links to the new API reference page. Added contextual links to specific API reference anchors throughout.
• Updated: docs.json — Added a "Reference" group near the top of the Base Chain nav, added api-reference to the Flashblocks section, and removed the duplicate reference entry.

Notes to reviewers

How has it been tested?
Locally

[cb-heimdall]

✅ Heimdall Review Status

Requirement	Status	More Info
Reviews	✅ 1/1	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

[ericbrown99]

Thanks for these quick changes! We should consider the trade-offs of having the sub-sections within this doc split out so that they are more discoverable vs a signle doc like this