Rework releases landing page into an overview

[jhlodin]

https://cockroachlabs.atlassian.net/browse/DOC-16115

• Rewrite conceptual content to be more "overview-friendly", easier to get introductory concepts
• Remove "downloads" dynamic content which is all duplicative with downloads available on individual release pages
• Add links directly to release pages from the releases table so it's easier to get to a specific release
• Fix issue where "latest production release" and "its major version" weren't aligned when generated
• Add version number dynamically to "Latest production release" and "Latest dev release" nav entries. May decide not to keep this.
• Update (un)supported cloud versions

TODO on this page, not in scope for this PR:

• Consolidate tables of releases in this page (and cloud releases page) into single table of all releases

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[github-actions]

Files changed:

• src/current/_includes/common/upgrade/overview.md
• src/current/_includes/releases/whats-new-intro.md:
  • releases/releases.md
• src/current/_includes/sidebar-all-releases.json
• src/current/cockroachcloud/upgrade-policy.md
• src/current/releases/index.md
• src/current/v26.1/restoring-backups-across-versions.md

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