docs: add v1.2.0 release docs

[mevan-karu]

Remove the v1.2.0-m.1, v1.2.0-rc.1, and v1.2.0-rc.2 pre-release snapshots and their config/changelog references now that v1.2.0 is GA. Snapshot current docs as the stable v1.2.x version (default, indexed); demote v1.1.x. Scaffold the v1.2-to-v1.3 upgrade guide. Update the changelog and release-and-support-process pages for v1.2.0.

[coderabbitai]

No actionable comments were generated in the recent review. 🎉

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📝 Walkthrough

Summary by CodeRabbit

• Documentation

  • Added an upgrade guide for moving from v1.2 to v1.3, noting no backward-incompatible changes and outlining the standard upgrade steps.
  • Added navigation to the new v1.2-to-v1.3 guide within the upgrade section.

• Release & Support

  • Updated the changelog to replace prior v1.2 pre-release entries with a finalized v1.2.0 release.

• Website / Versioning

  • Refreshed version tracking to surface v1.2.x documentation and updated the site announcement to reference v1.2.0.
  • Marked v1.2 as actively supported and removed the planned “in progress” v1.2 entry.

Walkthrough

Finalizes documentation and site versioning for the v1.2 release, updates release support tables and announcements, and adds navigation and instructions for upgrading from v1.2 to v1.3.

Changes

Release and upgrade documentation

Layer / File(s)	Summary
Finalize v1.2 release publication docs/changelog.md, docs/releases/release-and-support-process.md, docusaurus.config.ts, versions.json	Replaces v1.2 pre-release metadata with finalized v1.2/v1.2.x entries, marks v1.2 as actively supported, and updates the release announcement.
Add v1.2-to-v1.3 upgrade path docs/platform-engineer-guide/upgrades/*, sidebars.ts	Adds the v1.2-to-v1.3 upgrade page, overview entry, and sidebar link, including the standard CRD and Helm upgrade sequence.

Estimated code review effort: 2 (Simple) | ~10 minutes

Possibly related PRs

• openchoreo/openchoreo.github.io#792: Updates v1.2.0 release documentation and Docusaurus version metadata.
• openchoreo/openchoreo.github.io#756: Establishes versioned platform engineer upgrade-guide navigation.
• openchoreo/openchoreo.github.io#740: Updates v1.2 release support documentation.

Suggested reviewers: lakshanss

🚥 Pre-merge checks | ✅ 4 | ❌ 1

❌ Failed checks (1 warning)

Check name	Status	Explanation	Resolution
Description check	⚠️ Warning	The description is missing the required Purpose, Related Issues, and Checklist sections from the template.	Rewrite it with the template headings, add any related issue links, and complete the checklist items or note why they weren't run.

✅ Passed checks (4 passed)

Check name	Status	Explanation
Title check	✅ Passed	The title accurately summarizes the main change: adding v1.2.0 release documentation.
Docstring Coverage	✅ Passed	No functions found in the changed files to evaluate docstring coverage. Skipping docstring coverage check.
Linked Issues check	✅ Passed	Check skipped because no linked issues were found for this pull request.
Out of Scope Changes check	✅ Passed	Check skipped because no linked issues were found for this pull request.

✨ Finishing Touches

🧪 Generate unit tests (beta)

• Create PR with unit tests

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[coderabbitai]

Actionable comments posted: 1

🤖 Prompt for all review comments with AI agents

Verify each finding against current code. Fix only still-valid issues, skip the
rest with a brief reason, keep changes minimal, and validate.

Inline comments:
In `@docs/platform-engineer-guide/upgrades/v1.2-to-v1.3.mdx`:
- Around line 12-14: Update the upgrade procedure in the v1.2-to-v1.3 guide to
instruct operators to apply CRDs and run Helm upgrades only for planes installed
in their deployment. Preserve the control, data, workflow, and observability
ordering for planes that are present, and clarify that omitted optional planes
must be skipped.

🪄 Autofix (Beta)

Fix all unresolved CodeRabbit comments on this PR:

• Push a commit to this branch (recommended)
• Create a new PR with the fixes