Documentation: Update docs navigation structure to include explanations, tutorials, how-tos and API reference

[Ingerid]

Currently, the DHLAB documentation which is automatically built with sphinx and deployed to github pages only covers the API reference of the DHLAB python package.

The tutorials and how-to guides live in a different repo and get built with jupyter books: https://nationallibraryofnorway.github.io/digital_tekstanalyse/README.html

• Merge all the documentation pages into the same repo
• Change the landing page (index.md) navigation to display tiles for each group of information
• Write explanation pages for the main concepts (Corpus, collocation, concordance, clusters and N-grams)
• Change tile links from dh.nb.no to point to the new documentation pages.

Reference: https://docs.divio.com/documentation-system/

[Ingerid]

Flere punkter til oppfølging:

• Skille tydeligere mellom klasser og løse funksjoner
• Legge inn lenke til APIet i dokumentasjonssidene
• Skille tydeligere mellom tutorials ("slik funker dhlab-koden") og how-to-guides ("slik løser du problem x")
• Legge til docstrings der de mangler.
• Pass på at alle notebooks kjører med riktige avhengigheter