nbdev1 migration scripts and documentation needs to be improved

[jnareb]

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)

[jnareb]

Also I think that the document that mentions removing .gitconfig should also mention running nbdev_install_hooks.