docs: add release calendar page and references to it

docs: add release calendar page and references to it

Add a page that has information which release is expected when and
associated information.

Reference to it from the "Releasing process" and "Release notes" pages.

v2:
 - Add Andres for 17.0.5
 - Rework table format to include the branch (Eric)

Signed-off-by: Emil Velikov <emil.velikov@collabora.com>
Reviewed-by: Andres Gomez <agomez@igalia.com>
Reviewed-by: Eric Engestrom <eric.engestrom@imgtec.com>