docs: sharpen README

[karelxfi]

Summary

Rewrote the README to be sharper and more accurate against the actual workspace and source, without growing it for its own sake.

What changed and why

The previous README listed actors and crates that no longer match the code (e.g. "Transport crate / Messages crate" prose, "Gateway" rather than the current Portal terminology) and omitted several crates that exist today.

The new README is verified against:

• Cargo workspace (Cargo.toml, crates/*/Cargo.toml): seven crates documented exactly as named: sqd-network-transport, sqd-messages, sqd-assignments, sqd-contract-client, sqd-bootnode, sqd-keygen, sqd-observer.
• crates/transport/src: actor list and the Cargo feature gates (worker, portal, observer, pings-collector, logs-collector, portal-logs-collector, sql-client) taken from Cargo.toml [features] and lib.rs exports.
• crates/messages/proto/messages.proto and build.rs: confirms prost/protobuf schemas and the Worker/Scheduler/Portal/collector message flow (the proto comments call the role Portal, previously Gateway).
• crates/assignments/schema/assignment.fbs and lib.rs: FlatBuffers assignment format with builder/reader features.
• crates/contract-client/src: reads onchain registries/allocations over an RPC node via ethers.
• Dockerfile and .github/workflows/docker.yml: build targets and the three published images (subsquid/bootnode, subsquid/keygen, subsquid/observer); kept the keygen Docker example, updated the tag to :latest (confirmed present on Docker Hub).
• CLI flags from crates/transport/src/cli.rs and crates/observer/src/cli.rs for the Configuration table.
• rust-toolchain (1.89.0) and the protobuf-compiler build dependency from the Dockerfile.
• License: all crates declare AGPL-3.0-or-later; root LICENSE.md is AGPLv3. License line updated to match.

Crates are not published to crates.io (verified 404 in the sparse index), so no crates.io links were added.

Style compliance

• No marketing words, no em dashes, no emojis.
• "onchain" written as one word.
• Every link verified returning 200: docs.sqd.dev/en/network, sqd.dev, the network-architecture wiki, rust-libp2p, ethers-rs.

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