ENH: add next and prev document metadata to notebook meta

[mmcky]

Implements #317

This PR will add next and previous document metadata to the generated notebooks.

Such as:

 "metadata": {
  "date": 1591753649.837961,
  "filename": "index.rst",
  "kernelspec": {
   "display_name": "Python",
   "language": "python3",
   "name": "python3"
  },
  "next_doc": {
   "link": "index_toc",
   "title": "Table of Contents"
  },
  "title": "Python Programming for Economics and Finance"
 },

If metadata is valid:

1. next is referenced by nb.metadata.next_doc
2. previous is referenced by nb.metadata.prev_doc

[mmcky]

• [x] fix travis

[mmcky]

@DrDrij this implements the changes we discussed. Could you setup PR's for the theme's. If you would like to debug let's iterate on lecture-python-programming initially as it has lowest build times.

[mmcky]

@DrDrij once this passes travis I will merge and issue a new release of sphinxcontrib-jupyter

[DrDrij]

Perfect @mmcky I will update the themes over the weekend!