[helly25]

Prepare the documentation for toolchains_llvm 1.13.0. Documentation and release-template changes only; no runtime changes, catalogue updates, release tag, or publication.

Changes

• Add the 1.13.0 changelog entry and backfill 1.12.0 and 1.11.3 from their actual release contents. Keep LLVM 23.1.4 and the bzl 0.8.0 dependency upgrade under 1.12.0; attribute the new selector functionality to 1.13.0.
• Centralize the Cleanup of expression-based version selectors with explicit prerelease control #911 migration details in docs/migration-1.13.0.md, with concise warnings and direct links from README, CHANGELOG, and generated release notes. Keep the current configuration examples in README and the agreed 1.13.0 release number without describing this behavior change as backward-compatible.
• Separate prerelease eligibility from version constraints: the independent LLVM/linker opt-ins allow ordinary ranges without naming an RC. Show latest:>23.0,<=23.1 for following RCs through the final 23.1.0 release, <23.1 for excluding that final, and latest:major==23,minor==1 for following all 23.1.x patches. Exact pins remain unchanged.
• Explain LLVM's current undotted rcN spelling and SemVer ordering for both variants: rc10 < rc2, rc.2 < rc.10, and the mixed-spelling trap rc.10 < rc9. latest follows SemVer precedence, not release chronology; no normalization between spellings is performed.
• Record LLVM 23.1.5 from merged Add LLVM 23.1.5 distributions #932 in the 1.13.0 changelog: ten xz/zstd archives across Linux ARM64/x86_64, macOS ARM64, and Windows ARM64/x86_64. Update both installation examples to 23.1.5 with the bundled linker and call out the absence of a macOS x86_64 archive.
• Include the WORKSPACE aspect_bazel_lib 3.8.0 test-dependency upgrade from merged Update dependency aspect_bazel_lib to v3.8.0 #931.
• Correct the bzl documentation links to 0.8.0, the getenv/--repo_env explanation, the release-template pointer, and the .bazelrc typo.

LLVM 23.1.5 is part of the upcoming 1.13.0 release. Its catalogue update landed in #932; this PR documents it and updates the release installation examples.

Verification

• Bazel 9.3.0, Bzlmod: all 45 //toolchain/... tests passed with --nocache_test_results.
• Bazel 8.8.2, WORKSPACE: all 45 //toolchain/... tests passed with --nocache_test_results.
• Pinned bzl 0.8.0: all 24 @mboworks_bzl//bzl/versions:versions_tests passed on Bazel 9.3.0 with --nocache_test_results. Its existing tests cover dotted numeric identifiers, undotted lexical identifiers, and distinct mixed spellings.
• An isolated Starlark probe using our actual selector verified exclusive/inclusive ranges, RC-to-final selection, exclusion of later patches by <=23.1, the whole-23.1.x selector, explicit opt-in, stable eligibility, prerelease-only filtering, both RC spellings, and latest choosing rc9 over rc.10. This is a local diagnostic, not a newly committed regression test.
• Python: all 11 distribution-script tests and 14 wrapper tests passed.
• Trunk markdownlint, prettier, git-diff-check, and commit-time checks passed.
• Ran the existing release preparation script in an isolated temporary Git fixture, not the working checkout. Verified version/tag/commit substitution, generated archive checksum and prefix, both LLVM 23.1.5 installation examples, catalogue availability across Linux/macOS/Windows, migration text, the versioned migration-guide link, and guide inclusion in the generated archive. This smoke check does not publish a release or perform a new LLVM compile/link integration run.