Accomodate nbdev v1 users

[hamelsmu]

• make sure on nbdev website there's a very clear indicator of how to upgrade, or how to keep using the old version, including the home page.
• hide_input no longer works from nbdev v1, this has been replaced with #|echo: false
• Throw an error message with clear instrutions when someone tries to use nbdev v2 on a v1 project. We anticipate many people it's will upgrade to nbdev2 and use it on their nbdev1 project

cc: @jph00

[warner-benjamin]

Per Hamel, adding more issues attempting to convert a nbdev1 project to nbdev2.

• nbdev_new doesn't appear to respect the existing nbs_path and creates new notebooks in the root directory. It creates a _quarto.yml file in root and nbs_path. Unsure if both are needed.
• nbdev_new doesn't convert tst_flags from nbdev1's pipe format slow|cuda to nbdev2 format slow cuda. The nbdev1 format can cause broken pool errors in addition to test errors.

[jph00]

Per Hamel, adding more issues attempting to convert a nbdev1 project to nbdev2.

We're not planning to support using nbdev_new to upgrade an existing project. Instead we will endeavor to improve nbdev_migrate where possible -- although it won't be entirely automatic.

[jph00]

Here's the basic steps I took to upgrade ghapi:

# export LIBNAME=...

git rm Makefile
nbdev_migrate_directives
nbdev_export
git add $LIBNAME/_modidx.py
rm -rf docs

[[ $SHELL = *bash ]] && shopt -s globstar

perl -pi -e 's/#\|\s*hide_input/#| echo: false/' **/*.ipynb
perl -pi -e 's/#\|\s*hide_output/#| output: false/' **/*.ipynb
perl -pi -e 's/#\|\s*skip/#| eval: false/' **/*.ipynb
perl -pi -e 's/from nbdev.export import notebook2script/from nbdev import nbdev_export/' **/*.ipynb
perl -pi -e 's/notebook2script/nbdev_export/' **/*.ipynb
perl -pi -e 's/#\|?\s*all_slow/#| nbflags skip_exec/' *.ipynb

rm -f .gitconfig 
rm -f .git/hooks/post-merge

cat >>.gitignore <<EOF
_docs/
sidebar.yml
EOF

# change gh pages to gh-pages branch root
# move CNAME file

# change doc_path in settings.ini
# cp styles.css

nbdev_preview