Update latest release on GitHub

[anlambert]

Currently, the latest Tulip release on GitHub still points to the 5.1 version but last Tulip release is 5.3.

This must be updated and the 5.3 release notes should also be added.
It would also be a nice idea to upload Tulip binaries on GitHub too in case a downtime happens on SourceForge.

And for god sake, please add a link to that repository in the main Tulip website and on the Sourceforge page (#46).

[anlambert]

Just a nitpick on the updated latest release: the release should be named "Tulip 5.3.0" instead of "Tulip latest release". The latest release on Github is just a pointer on a git tag (here tulip_5_3_0).

Also copying and pasting the release notes from the main Tulip website would be a plus.

[anlambert]

Please rename the latest release title to "Tulip 5.3.0" (this is how it should be named, see OCaml latest release for instance).