releasing: update for the new release process

A few things have changed:

- Meson is used instead of autotools
- Wayland and Weston releases are not synchronized anymore
- Artifact deployment happens via wayland.freedesktop.org's Git repo

While at it, also convert the file to Markdown. Instructions to locally install
Xwayland/libinput have been dropped.

Signed-off-by: Simon Ser <contact@emersion.fr>