Concept: blackholed account

[mDuo13]

Add a brief concept article about "blackholed" accounts, which should cover:

• What it means to blackhole an account (basically, to publicly throw away the keys)
• Why people do it (to prove they can't use an account any more, often relating to issuing more tokens)
• How to recognize a blackholed account (master key disabled, no multi-signing list, and regular key is a known black hole)
• Downsides / problems with blackholing an account (you can't do anything with it, so all XRP it holds is lost, if you screwed up any settings or anything you can't fix those later, etc.)
• Edge cases and caveats (e.g. Offers placed in the DEX before blackholing can result in more tokens being issued after blackholing as those offers get taken. Similarly if the account placed offers to sell XRP before it was blackholed, those offers can be taken after it was blackholed, so that's a way that XRP could "escape" the blackhole)

[mDuo13]

Related: there's a code sample for how to blackhole an account (added by https://github.com/XRPLF/xrpl-dev-portal/pull/1684 ) but it's in the misleadingly-labeled folder delete-account/.

(Deleting an account is pretty different than blackholing it; the bounty for that one was unfortunately vague about which one to do)