Document the existence of FlexASIO Fluent

[dechamps]

FlexASIO Fluent is a brand new FlexASIO GUI app by @ramiro-uziel that just came out. I should check it out and link it from FlexASIO docs as an alternative to FlexASIO GUI.

Also: according to its README, FlexASIO Fluent hijacks the FlexASIO GUI registry entry so that it opens as the FlexASIO control panel, which is… arguably not the most polite thing to do. Maybe a cleaner approach could be arranged.