docs: update docs/architecture/workflows.md and release-process.md for Pre-Release-As trailer feature

[github-actions]

Component

Documentation

Bug Description

The Pre-Release-As: commit trailer feature was added to release-prerelease-pr.yml in commit 310c68c (PR #1565) but is not documented in either of the two mapped documentation files:

• docs/architecture/workflows.md — describes release-prerelease-pr.yml only as "Pre-release companion PR management" with no mention of the trailer override capability
• docs/contributing/release-process.md — documents release-please's Release-As: mechanism but has no equivalent section for the pre-release Pre-Release-As: trailer

The implementation in .github/workflows/release-prerelease-pr.yml (lines 81–87) honors a Pre-Release-As: commit trailer to override the computed pre-release version, mirroring the stable pipeline's Release-As: trailer behavior.

Expected Behavior

• docs/architecture/workflows.md should document the Pre-Release-As: trailer as a version override mechanism for the release-prerelease-pr.yml workflow, alongside the existing description of that workflow.
• docs/contributing/release-process.md should include a note (similar to the Release-As: documentation) explaining that maintainers can add Pre-Release-As: X.Y.Z as a commit trailer to force a specific pre-release version.

Steps to Reproduce

1. Review commit 310c68c — "feat(workflows): add Pre-Release-As trailer override to pre-release w..."
2. Check .github/workflows/release-prerelease-pr.yml lines 81–87 for the implementation
3. Search docs/architecture/workflows.md and docs/contributing/release-process.md for Pre-Release-As — no results found

Additional Context

• Changed workflow file: .github/workflows/release-prerelease-pr.yml
• Documentation files to update: docs/architecture/workflows.md, docs/contributing/release-process.md
• The feature mirrors release-please's Release-As: trailer, so the documentation pattern from that feature is a good reference

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