docs: Expand and update release howto a bit.
docs: Expand and update release howto a bit.

These are notes I made while the first 14.x release today.
* Expand a bit on the Xcode-login bugs and the workarounds.
* Clarify what releases on GitHub are about.
* Update to reflect how I've been handling the alpha channel.
* Small other updates.

vishwesh3/zulip-mobile,vishwesh3/zulip-mobile,vishwesh3/zulip-mobile,vishwesh3/zulip-mobile