docs: backport testnet v4.2.0 update + AI tooling trim (backport #22744)

[AztecBot]

Backport of #22744 to backport-to-v4-next-staging.

Summary (from upstream PR)

• Bump developer_version_config.json mainnet/testnet to v4.2.0.
• Remove aztec-claude-plugin marketplace install block and its row from the Aztec/Noir tool reference table in the AI tooling guide (current source + the closest matching versioned snapshot).
• Replace em-dashes with colons per style guide in getting_started_on_testnet.md (current + versioned).
• Drop the "Use Case Suitability" table from docs/docs/networks.md.

Conflict resolution

The automatic cherry-pick failed on:

• docs/developer_version_config.json — HEAD had mainnet: v4.2.0-aztecnr-rc.2, testnet: v4.1.0-rc.2. Resolved by taking the incoming v4.2.0 for both (the upstream PR's intent).
• docs/docs/networks.md — The "Use Case Suitability" table existed only on HEAD. Resolved by dropping the table per upstream PR.

Notes for reviewer

• The upstream PR edited docs/developer_versioned_docs/version-v4.2.0/ai_tooling.md and getting_started_on_testnet.md. That path does not exist on this branch, so git's rename heuristic auto-merged the changes into version-v4.1.0-rc.2/ (the closest match). version-v4.2.0-aztecnr-rc.2/ai_tooling.md still references aztec-claude-plugin — flag if you'd prefer the cleanup applied there instead.
• Backport follows the 3-commit structure (cherry-pick with markers, conflict resolution); no build-fix commit was needed since this is docs-only.

Test plan

• yarn build in docs/ completes without errors
• Rendered testnet install command shows VERSION=4.2.0
• AI tooling page no longer references aztec-claude-plugin

ClaudeBox log: https://claudebox.work/s/c8425044cd859e4d?run=1