nbdev1 migration scripts and documentation needs to be improved

I am doing a migration from nbdev1 to nbdev2 for my project, following https://nbdev.fast.ai/migrating.html document, and I have encountered the following problems.

- The nbdev_migrate script has some problems where the page description (which in nbdev1 was done as a quote just below h1 header) spanned more than one line; only the first line was copied.
- The nbdev_migrate script automatically converts directives, but it does not convert the old `notebook2script` calls
- In _"Update directive names"_ there is no need to do the convert directives, as it is handled by nbdev_migrate script
- With the exception of `git add $LIBNAME/_modidx.py` the migration documentation does not say anything about adding (or not) those newly downloaded files to the repository with `git add` - should it be done (and why), should it not be done (and why)?
- Instruction on how to update GitHub Pages settings is done **3** times.
- I think instead of using perl one can simply use sed in those examples (though I am not exactly sure which is easier for an ordinary user)

Inizia da migrating.html e dallo script nbdev_migrate, verificando i passaggi di migrazione elencati, la gestione delle direttive, le descrizioni delle pagine su più righe e le vecchie chiamate a notebook2script. Esamina le indicazioni relative a git add, alle impostazioni di GitHub Pages e a perl rispetto a sed. Il lavoro è completato quando lo script e la documentazione della migrazione coprono in modo coerente ogni caso segnalato senza istruzioni duplicate.

Scritto dal modello di indicizzazione a partire dal testo della issue.