Make the index page include most of the README programmatically

[daslu]

The index page of the docs and the README have a lot in common.
Currently, we maintain them separately and need to keep them in sync.
https://scicloj.github.io/noj/
https://github.com/scicloj/noj/

A more consistent solution will be to programmatically include the README in the index page.

See, for example, the approach in Wolframite and Tablemath:
https://github.com/scicloj/wolframite/blob/main/notebooks/index.clj
https://github.com/scicloj/tablemath/blob/main/notebooks/index.clj