Reorganize and simplify releases nav

[jhlodin]

https://cockroachlabs.atlassian.net/browse/DOC-16117

• Remove "Staged release process" page which is an outdated artifact of the release policy change
• Move "Technical advisories" down to reference section which is more appropriate than releases
• Rework "Unsupported versions" into a "downloads archive" which is more accurate for the page's contents, and aligns with the long-term plan of removing archived CRDB binary downloads off of docs altogether
• Move "Archived documentation" down to reference section which is more appropriate than releases
• Add redirects as needed

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Files changed:

• src/current/_data/redirects.yml
• src/current/_includes/releases/staged-release-policy.md:
  • releases/releases.md
• src/current/_includes/sidebar-all-releases.json
• src/current/_includes/sidebar-releases.json
• src/current/_includes/v23.1/sidebar-data/reference.json
• src/current/_includes/v23.2/sidebar-data/reference.json
• src/current/_includes/v24.1/sidebar-data/reference.json
• src/current/_includes/v24.2/sidebar-data/reference.json
• src/current/_includes/v24.3/sidebar-data/reference.json
• src/current/_includes/v25.1/sidebar-data/reference.json
• src/current/_includes/v25.2/sidebar-data/reference.json
• src/current/_includes/v25.3/sidebar-data/reference.json
• src/current/_includes/v25.4/sidebar-data/reference.json
• src/current/_includes/v26.1/sidebar-data/reference.json
• src/current/_includes/v26.2/sidebar-data/reference.json
• src/current/releases/downloads-archive.md
• src/current/releases/index.md
• src/current/releases/staged-release-process.md

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[taroface]

A couple minor suggestions. However, I'm not sure I agree with

Move "Technical advisories" down to reference section which is more appropriate than releases

The TAs are release-specific contingencies that someone contemplating an upgrade may want to see. They also don't feel consistent with general "Reference" material. The way that they're presented feels reminiscent of a changelog, since this is a running list of notices. I suggest leaving this under Releases for now.

[taroface]

Titles have to use title case:

Suggested change

	title: CockroachDB downloads archive
	title: CockroachDB Downloads Archive

[taroface]

Something like this feels more intuitive IMO - "downloads archive" suggests to me that all releases are found here.

Suggested change

	## CockroachDB downloads archive
	## Archived releases

[taroface]

Suggested change

	A full archive of CockroachDB major versions that are no longer supported is available on the [CockroachDB downloads archive]({% link releases/downloads-archive.md %}) page.
	To access CockroachDB versions that are no longer supported, refer to [CockroachDB Downloads Archive]({% link releases/downloads-archive.md %}).

[jhlodin]

A couple minor suggestions. However, I'm not sure I agree with

Move "Technical advisories" down to reference section which is more appropriate than releases

The TAs are release-specific contingencies that someone contemplating an upgrade may want to see. They also don't feel consistent with general "Reference" material. The way that they're presented feels reminiscent of a changelog, since this is a running list of notices. I suggest leaving this under Releases for now.

@taroface I think the context in which someone wants to know about a technical advisory is when the advisory is posted, rather than at a time when they're considering upgrading more broadly, which is why I don't generally think of TAs as release material. My proposal here is that it's a low-risk opportunity to further simplify the releases nav, which is already dense with the variety of release notes captured.

Case in point, Starburst docs bundle technical advisories in with other high-level security information despite having advisories associated with affected and mitigated product versions.

Let me know what you think, that's my argument but if you disagree I'm comfortable reverting this one.