Move and update release documentation from these wiki pages to pmaports docs: * https://wiki.postmarketos.org/wiki/Releases * https://wiki.postmarketos.org/wiki/Creating_a_release_branch#Timeline Signed-off-by: Oliver Smith <ollieparanoid@postmarketos.org> Part-of: <https://gitlab.postmarketos.org/postmarketOS/pmaports/-/merge_requests/8486>