Add SEP-45 pubnet contract deployment

[philipliu]

This adds a pubnet instance of the SEP-45 web auth contract.

The Wasm was built as part of a sep45-reference release but is currently showing up as not verified, both the CLI and the lab, due to an issue with SEP-55 described in stellar/stellar-cli#2358

The build can be verified manually using the GH CLI until it's resolved.

❯ gh attestation verify --owner stellar web_auth.wasm
Loaded digest sha256:3c8d0b8b347752e57abe0b50380401ca8f5793bc971b685fd072571bbf5d54cc for file://web_auth.wasm
Loaded 1 attestation from GitHub API

The following policy criteria will be enforced:
- Predicate type must match:................ https://slsa.dev/provenance/v1
- Source Repository Owner URI must match:... https://github.com/stellar
- Subject Alternative Name must match regex: (?i)^https://github.com/stellar/
- OIDC Issuer must match:................... https://token.actions.githubusercontent.com

✓ Verification succeeded!

The following 1 attestation matched the policy criteria

- Attestation #1
  - Build repo:..... stellar/sep45-reference
  - Build workflow:. .github/workflows/release.yml@refs/tags/v0.1.3
  - Signer repo:.... stellar/sep45-reference
  - Signer workflow: .github/workflows/release.yml@refs/tags/v0.1.3

[Copilot]

Pull request overview

This PR adds a pubnet deployment address for the SEP-45 web authentication contract to the documentation. The contract has been deployed to pubnet as part of the sep45-reference v0.1.3 release.

Changes:

• Updates the Implementations section in SEP-0045 to document the pubnet contract address

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