OCaml 4.14.0 release

[Octachron]

This is the draft update for the 4.14.0 release.

This PR adds:

• an entry in site/release/index.md and site/release/index.fr.md
• a release page for 4.14.0
• the 4.14.0 version of the reference manual (with the Changes notes)

It also update the information about the latest version in:

• the front page
• the version_selector script
• the symbolic link to the latest version of the manual and standard library documentation

This is a draft PR because I shall update the precise release date on the day before the release. Moreover, the list of highlight is still open to discussion.

[Octachron]

I have updated the PR, it can now be merged as soon as the compiler is available on opam.

[avsm]

compiler's on opam, merging away!