v3 site changes

[MarkAckert]

Completed:

• v3 data in releases.yml, index and download page updates
• Instructions on how to verify artifacts with sigstore
• Sigstore verification for all download artifacts
• SBOM download directions
• Add IntelliJ Explorer download link for v3 only (went with markeplace)
• Updates to Zowe Explorer title and description on downloads page: Zowe Explorer for VSCode to disambiguate it with IntelliJ.
• Compatibility and conformance statement on downloads page

Fixes:

• Empty currentTime caused Next release section to show the wrong value

Not modified:

• Should vNext and links to vNext need to be removed or put into cold storage ? i.e. keep them in the repo but disable access through the website
  • Leaving this as-is for now, the release schedule is useful.

Will address: #830, #855

Signed-off-by: MarkAckert [email protected]