docs: refresh REST and public testnet guidance

[pgarciagon]

Summary

This PR supersedes #230 by applying its still-relevant REST and public-testnet
improvements on top of the current dev branch.

• document live mainnet and public-testnet REST/JSON-RPC endpoints
• add a verified transaction-prepare example and clarify that prepare does not
  sign or broadcast
• update the embedded Swagger server for the Koinos Foundation public testnet
• replace the retired testnet/faucet guidance in the legacy Developers pages
• make offline-signing guidance retrieve the resettable testnet chain ID instead
  of copying a fixed value
• add warnings that historical serialized transactions, nonces, signatures,
  and chain IDs must not be reused
• update the Common Tasks REST and multisig connection guidance

The cross-chapter Developers/Common Tasks scope is intentional and explicitly
authorized for this #230 replacement. Current Getting Started, Contracts, and
Interacting pages were preserved because they already contain newer executable
examples and current testnet guidance.

Current-source validation

Checked against:

• koinos/koinos-testnet README and faucet documentation
• https://api.koinos.io/v1/chain/head_info
• https://api.koinos.io/v1/transaction/prepare
• https://api.koinos.io/jsonrpc
• https://api.koinos.io/swagger
• https://testnet.koinosfoundation.org/health
• https://testnet.koinosfoundation.org/v1/chain/head_info
• https://testnet.koinosfoundation.org/jsonrpc

Verification

• npm run examples:verify
• npm run examples:syntax
• npm run docs:links
• python3 -m json.tool docs/developers/swagger.json
• docs-venv/bin/mkdocs build
• git diff --check
• browser review of Developers REST, Developers Testnet, Common Tasks REST,
  multisig, and offline-signing pages
• browser click-through from offline signing to the dynamic chain-ID guidance

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