doc: Add doc/develop/release_cycle.rst

Migrate the RelaseCycle wiki page to Sphinx.  In terms of visible
changes, we stop having a dynamic countdown to when the release is.  And
we drop the year-based statistics, that were not being kept up to date.
For the moment, we only link to statistics for v2022.07 but will add
back the historical data in a subsequent patch.

Cc: Heinrich Schuchardt <xypron.glpk@gmx.de>
Signed-off-by: Tom Rini <trini@konsulko.com>