Update and tidy data analysis section

[paigem]

Reminder for myself to:

• remove the nesting of the one file (or add words to the overview)
• add a page for Dask/Xarray
  • link to other resources

[paigem]

Update: Intro text has been added to the computation section (see #90)

Tasks still open:

• add a page for Dask/Xarray
• add a page for AI/ML