Rewrite "Reserves" article to be friendlier to new readers

[mDuo13]

The "Reserves" article is a common entry point to the docs because certain sites (Gatehub) link to it as an explanation. Feedback on this article is lower than most, probably because people getting to it want an easier answer to where their XRP went.

The introduction to the article should serve this audience a little better and, in general, be accessible to people who don't know all that much about the XRP Ledger. It should also probably be updated to mention account deletion as an option to get (some of) the reserve back. Maybe the owner reserves section could be made clearer with a table or something too.