[DOCUMENTATION] Improve the release instruction

[bchapuis]

• Verify the email templates. The download url should be changed to: https://downloads.apache.org/incubator/baremaps/[version]/
• Add instruction to sign the artifacts with personal key
• Revise the steps
• Fix release actions (github) that don't get triggered due to wrong tag regex
• Add instructions to update the download page on the website