Genericise recipes

Something raised in #429 (sparked by comments here https://github.com/amiaopensource/ffmprovisr/pull/422#issuecomment-622791512 and here https://github.com/amiaopensource/ffmprovisr/pull/428#issuecomment-636422487)
was that a bunch of existing recipes could be elaborated and genericised
* rewritten to use placeholder variables (this is preferable), and/or
* fuller explanations given to any hardcoded variables

An example of this is the [Fade audio & video streams recipe](https://github.com/amiaopensource/ffmprovisr/blob/b04c0723b5e9b1e6ab9bc28c3560a8996d4426c4/index.html#L731).

I'm interested in having a look over the recipes, but it might not be soon, so anyone is welcome to pick this up!