ci: Build docs in PRs and in private repos

[mathbunnyru]

High Level Overview of Change

This:

• builds docs in PRs correctly (including PRs created from forks)
• makes it work in private repos, where ubunu-latest is a runner with only 2 CPUs

Context of Change

Type of Change

• Bug fix (non-breaking change which fixes an issue)
• New feature (non-breaking change which adds functionality)
• Breaking change (fix or feature that would cause existing functionality to not work as expected)
• Refactor (non-breaking change that only restructures code)
• Performance (increase or change in throughput and/or latency)
• Tests (you added tests for code that already exists, or your new feature included in this PR)
• Documentation update
• Chore (no impact to binary, e.g. .gitignore, formatting, dropping support for older tooling)
• Release

API Impact

• Public API: New feature (new methods and/or new fields)
• Public API: Breaking change (in general, breaking changes should only impact the next api_version)
• libxrpl change (any change that may affect libxrpl or dependents of libxrpl)
• Peer protocol change (must be backward compatible or bump the peer protocol version)

[Copilot]

Pull request overview

This PR updates the documentation publishing workflow so it selects a smaller parallelism reduction when running in private repositories, accommodating GitHub-hosted runner CPU limits to avoid over-subtracting cores during the docs build.

Changes:

• Set NPROC_SUBTRACT dynamically based on whether the repository is private (1) or not (2).
• Add inline comments and a reference link explaining the private-repo runner CPU constraint.

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[godexsoft]