docs: download link for standalone app

[norwnd]

Based on https://github.com/decred/dcrdex/pull/2456, blocked by https://github.com/decred/dexweb/issues/41 (hence PR is a draft).

See rendered version.

It will probably be useful to have the https://dex.decred.org/#downloads link in setup section(s), once it's ready to distribute standalone apps: https://github.com/decred/dexweb/issues/41.

[xaur]

IMO it is confusing to have two places to download the same standalone app from.

Another thing possibly for a separate PR is I would help users choose between dexc-windows-... and dexc-tray-windows... in that top list of download options. I talked to one user who got confused by this choice.