Document new grub config file hooks

[zeha]

Yes, not sure where exactly though :) Should we add this to docs/grml-live.8.adoc? Maybe the nice info from @Zugschlus from https://lists.mur.at/hyperkitty/list/grml-devel@ml.grml.org/thread/XKVFTSTYFYDN3JS3ILUFICNQK74O456T/ could be integrated somehow even? 🤔

Originally posted by @mika in #547 (comment)

[Zugschlus]

I am unsure whether this is worth documenting in detail. People who want to go down on this level are likely to check out git and see the code anyway. Documentation of such things is usually not exact enough to make proper use of them without looking at the code. Maybe something along the lines of "the grub configuration is instrumented with hooks for remasterered images to use, please see the code at if you want to use them" is enough.

The most natural place inside the grml-live.8 would be the FAQ.