Prepare a what's new page for the 3.1.0 release

[andreyaksenov]

Product: Tarantool
Since: 3.1.0
Root document: https://www.tarantool.io/en/doc/latest/release/

Main steps

• Prepare a what's new page before the release: https://www.tarantool.io/en/doc/latest/release/3.1.0/
• Consider adding a section about the experimental.connpool module #4129.
• Add a release date and a link to GitHub after the release.
• Add a new release to the https://www.tarantool.io/en/doc/latest/release/ page.