Octavo integration

[RensDimmendaal]

This PR adds octavo as an optional backend for nbdev-docs. The migration instructions are added to the octavo notebook's module docstring.

The migration is probably not perfect, because quarto has many features and I dont know which features other nbdev projects all use.

To test it I recommend reading through the notebook of course, but also to test it on another nbdev project's docs. I've ran it on nbdev's own docs and basedpl's docs for example.

I know the number of changes looks very large, but it's mainly explanations and tests in the octavo notebook.

$ g d main --stat
 nbdev/__init__.py           |    1 +
 nbdev/_modidx.py            |   23 +
 nbdev/doclinks.py           |   18 +-
 nbdev/octavo.py             |  288 +++++++++++
 nbdev/processors.py         |    4 +-
 nbdev/quarto.py             |   25 +-
 nbs/api/05_doclinks.ipynb   |   18 +-
 nbs/api/10_processors.ipynb |    4 +-
 nbs/api/14_quarto.ipynb     |   43 +-
 nbs/api/20_octavo.ipynb     | 1600 +++++++++++++++++++++++++++++++++++++++++++++++++++++++++++
 pyproject.toml              |    4 +-
 11 files changed, 1996 insertions(+), 32 deletions(-)