docs: fix links

README.md

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 A Python package to compute and visualize observation metrics for periodic events.
 
 In Astronomy, *portrait* is useful to answer the following questions:
-- How much of an orbit with period $P$ has been observed given certain observation times? ([tutorial](./docs/coverage.ipynb))
-- What period $P$ matches with this list of events? ([tutorial](./docs/periodmatch.ipynb))
-- How a certain target must be observed to cover all orbits with periods lower than $P$ days? 
+- How much of an orbit with period $P$ has been observed given certain observation times? ([tutorial](https://portrait.readthedocs.io/en/latest/coverage.html))
+- What period $P$ matches with this list of events? ([tutorial](https://portrait.readthedocs.io/en/latest/periodmatch.html))
+- How a certain target must be observed to cover all orbits with periods lower than $P$ days?