docs: add v1.2.0 release notes

[discord9]

What changed

Add blog/release-1-2-0.md with the v1.2.0 release notes, using the established release-post format and the same body intended for the GreptimeDB GitHub Release.

• Curate highlights for JSON2 v2, Prometheus Remote Write v2/native histograms, query efficiency, Splunk HEC, and Flow runtime status.
• Cover the embedded Dashboard upgrade separately.
• Document configuration, local-file access, beta-format/edition, and legacy JSON2 upgrade caveats.
• Include the full minor-release changelog from v1.1.0 through the immutable v1.2.0 release commit, subtracting changes already shipped in v1.1 patches and rebuilding contributors.

Release tracking: GreptimeTeam/greptimedb#9015
Release: https://github.com/GreptimeTeam/greptimedb/releases/tag/v1.2.0

Scope

• Documentation versions: v1.2.0 release blog only; no version snapshots, variables, navigation, or site-root promotion.
• Languages: English shared release post, following existing release-blog practice; identical content is used for GitHub. No localized manual pages are changed.

Verification

• Release-source audit: exact 336-commit range ending at 29b743ab95568a47884ef0e8c42d579f0b0222e9; 270 included PRs and 24 non-bot contributors; already-released patch changes excluded.
• Published Linux amd64 v1.2.0 binary: all five SQL example statements succeeded; JSON2 query returned the expected typed values; Flow commands returned the expected schema.
• git diff --check: passed.
• typos --config .typos.toml blog/release-1-2-0.md: passed.
• DOC_LANG=en pnpm check:links: passed (full final note).
• DOC_LANG=zh pnpm check:links: passed (full final note).
• Local frontmatter-stripped blog and GitHub candidate body: byte-for-byte equal.
• Version-pinned migration-guide source and locally built route verified; live docs HTTP verification returned 403 in this environment.
• GitHub/body synchronization: passed .github/scripts/check-release-note-sync.sh df4ca8b1a074d7ca583484cf223c43e398a2bc59 after publishing the identical body. GitHub remains Pre-release; Latest remains v1.1.4. No release-state changes are part of this PR.

Checklist

• I verified the content against the applicable GreptimeDB version.
• I updated the relevant documentation versions and languages, or explained why not.
• I checked changed links and anchors.
• I updated navigation when the document structure changed. (Not applicable: release-blog entry only; automatic blog routing.)

[cloudflare-workers-and-pages]

Deploying greptime-docs with    Cloudflare Pages

Latest commit:	52a33fa
Status:	✅  Deploy successful!
Preview URL:	https://58d191cf.greptime-docs.pages.dev
Branch Preview URL:	https://docs-release-v1-2-0-notes.greptime-docs.pages.dev

View logs

[killme2008]

The migration-guide link in the first breaking change returns 404:

404  https://docs.greptime.com/1.2/user-guide/deployments-administration/migrate-local-sql-file-access
200  https://docs.greptime.com/user-guide/deployments-administration/migrate-local-sql-file-access

1.2 became the site root version in #2840, and the root version has no path prefix, so /1.2/ no longer resolves. Dropping the prefix fixes it. Worth doing before the body is synced to the GitHub Release.

The rest of the highlights check out against the v1.2.0 tag — including the distributed start_time/uptime_seconds NULL note, which matches FlowStateValue dropping start_time_map.

[killme2008]

LGTM