docs: split release install by platform, warn on draft links

[0xKT]

Summary

Two fixes to how a release is presented, both found while cutting 0.1.10.

Install / Upgrade in the release notes. The template shipped a single
curl | bash line, so every release page told native Windows readers nothing
about install.ps1 and told existing users nothing about upgrading -- even
though a release page is mostly read by people who already run Raven. Install
now has one block per platform, mirroring README: install.sh for
Linux / macOS / WSL2, install.ps1 for native Windows plus the PowerShell 5.1
direct-URL note (the short URL fails there with Permanent Redirect). A new
Upgrade section gives raven upgrade and its three limits: latest stable only,
so a pre-release still needs the installer (rc tags are published with
--prerelease and never become /releases/latest); editable checkouts are
never overwritten; on native Windows an external helper finishes the job.

The draft-link trap. GitHub addresses a draft release as
releases/tag/untagged-<hash>, even when the tag already exists -- CI only runs
after the tag is pushed. Publishing moves the release to releases/tag/vX.Y.Z
and leaves the old URL serving its own stale page with no redirect. With 0.1.10
that draft link had been shared, so after publication two readers hit the stale
page and concluded, once that the tag was never pushed and once that publishing
had not taken effect; both were wrong. RELEASING.md now states this at the
publish step, and the release job prints the draft URL next to the public one in
its step summary, so whoever cuts the release has the correct link at hand
instead of copying the address bar. The notes skeleton in RELEASING.md is also
brought in line with the new template.

Type

• Fix
• Feature
• Docs
• CI / tooling
• Refactor
• Other

Verification

• sed "s/__VERSION__/0.1.11/g; s/__TAG__/v0.1.11/g" .github/release-notes-template.md
  -> both placeholders substituted, no leftover __ markers

• Applied the same Install / Upgrade section to the pending 0.1.10 draft release
  and reviewed the rendered GitHub page before committing; that draft's summary,
  Highlights, Release Status, and Notes were left untouched

• release.yml parsed with yaml.safe_load, and the modified run: block
  checked with bash -n

• Every CI check reproduced locally, all green: npx commitlint --from origin/main --to HEAD --config commitlint.config.cjs,
  uv run --extra dev python scripts/check_commit_messages.py origin/main..HEAD,
  uv run --extra dev python scripts/check_large_files.py origin/main..HEAD,
  uv run pre-commit run --from-ref origin/main --to-ref HEAD, make lint-python,
  make test-python (5143 passed, 30 skipped), plus
  scripts/check_pr_title.py and scripts/check_pr_body.py on this title and body

• Relevant tests pass locally

• Relevant lint / type checks pass locally

• User-facing docs or screenshots are updated when needed

Risk

Documentation, plus one added step-summary write in the release workflow. No
runtime code path changes; already-published release notes are unaffected. The
workflow change only adds output -- the release-creation logic is untouched, and
the else branch (release already published) keeps its existing behaviour.
Rollback is reverting the two commits.

• Security impact considered
• Backward compatibility considered
• Rollback path is clear for risky changes

Related Issues

N/A