docs: mainnet operator notes — versions table row and network selection guide

[osr21]

Summary

With Arc mainnet approaching, the operator docs only describe testnet. This PR adds the mainnet facts that are already verifiable from this repository — nothing speculative, no invented endpoints.

docs/installation.md — adds an Arc Mainnet row (v0.7.0 or later) to the Versions table, with a note carrying the version floor and the v0.7.0 libp2p peering implication over from BREAKING_CHANGES.md, where operators reading the installation page currently won't see it.

docs/running-an-arc-node.md — adds a "Choosing a network" section after the intro:

• testnet/mainnet table: --chain value, chain ID, minimum version
• both chainspecs are bundled in the binaries (arc-mainnet is a named chainspec since v0.7.0 — crates/execution-config/src/chainspec.rs registers it, genesis at assets/mainnet/genesis.json)
• mainnet notes: v0.7.0 floor + no staged rollouts across the boundary (per BREAKING_CHANGES), snapshot bootstrap via --chain=arc-mainnet once mainnet snapshots are published, and a warning not to reuse the guide's testnet RPC/follow endpoints on mainnet

What this PR deliberately does NOT do

• Does not touch the existing testnet row (v0.6.0 → v0.7.3 is docs: update Arc Testnet version to v0.7.3 in installation.md #226's claimed fix; this PR only adds adjacent lines, though whichever merges second will need a trivial rebase of that hunk).
• Does not list mainnet RPC or snapshot URLs — none are publicly resolvable yet (I checked; snapshots.arc.network has no arc-mainnet manifest and no mainnet RPC hostname answers). The text says "once published" rather than inventing endpoints. Happy to follow up with concrete URLs when Circle publishes the mainnet infrastructure.

Verification

• Chain ID 5042 and the arc-mainnet chainspec name verified against crates/shared/src/chain_ids.rs and crates/execution-config/src/chainspec.rs on main.
• Version floor and peering rules verified against BREAKING_CHANGES.md / CHANGELOG.md v0.7.0 entries.
• Both files render cleanly; links checked relative to the docs directory.

[ZhiyuCircle]

Hi @osr21,

Thank you for your interest in contributing to Arc Node, and apologies for the delay in getting back to this PR.

We're closing out the pull request backlog that predates our current contribution policy. This PR is being closed because the number it references is a pull request, not an issue. All PRs must reference an existing issue using the format Closes: #XXX, and the author must be assigned to that issue before the PR is opened.

This is not a judgement on the change itself. If you'd still like to land it:

1. Open an issue describing the problem, or find the existing one
2. Comment on the issue requesting assignment, and wait for maintainer approval
3. Open a fresh PR once you have been assigned

Please see CONTRIBUTING.md for details. Thanks again for taking the time to contribute.