Docs: Inconsistent release notes titles, headers, and a broken snippet import

[kiran1287]

Problem

Release notes pages across v1.12.x, v1.13.x, and v2.0.x-SNAPSHOT had inconsistent naming:

• Sidebar titles and in-page H1s mixed "X.Y Release" (singular, sidebar) with "X.Y Releases" (plural, H1), and the current-version page for each stream was oddly named all-releases.mdx while every older minor version had its own <version>-release.mdx file.
• Frontmatter title: fields carried a redundant trailing suffix (e.g. 1.10 Release | OpenMetadata 1.10 Releases) instead of ending cleanly at OpenMetadata.
• The docs.json nav group was labeled "All Releases" instead of "All Release Notes", inconsistent with the rest of the naming.
• v1.12.x/releases.mdx imported a nonexistent snippet, /snippets/releases/1.2.14.mdx (typo, missing a digit — should be 1.12.14.mdx), which would have rendered a broken/blank latest-release page for v1.12.x.

Fix

• Renamed all-releases.mdx to <version>-release.mdx per version line, matching the existing pattern for older minors
• Standardized sidebar titles and H1s to X.Y Release Notes (Older Release Notes for the archive page, Latest Release Notes for the two pages that track the current shipping version)
• Truncated frontmatter titles to end at OpenMetadata
• Renamed the "All Releases" nav group to "All Release Notes" in docs.json
• Fixed the broken 1.2.14.mdx → 1.12.14.mdx snippet import

Fixed by #394.