Draft: Add some documentation for the lbexec-type options

I recently discovered the extra_args option that can be used in the yaml configuration and find it is really useful so thought I would try to add something about it in the docs.

While doing this, I realised there is not really much information about what else can be specified with the lbexec-type of options so thought I would add a little about that too but in some cases my understanding is not very complete!

I started adding a bunch of the extra_options keys but then realised that they are probably not really AProd related (but couldn't yet find some other LHCb documentation to link to for the possible choices).