Add a tutorial section

[xiki-tempula]

Fix #382

• add link to docs in README
• add tutorial section to docs

See https://github.com/openjournals/joss-reviews/issues/6934 :

• providing a single example case with input file, e.g. simple example, e.g., a self-contained jupyter notebook, to walk through the main functionalities.
• add link to docs inside README

[codecov[bot]]

Codecov Report

All modified and coverable lines are covered by tests :white_check_mark:

Project coverage is 98.83%. Comparing base (f40b89b) to head (7031577).

Additional details and impacted files

```diff @@ Coverage Diff @@ ## master #379 +/- ## ======================================= Coverage 98.83% 98.83% ======================================= Files 28 28 Lines 1895 1895 Branches 407 407 ======================================= Hits 1873 1873 Misses 2 2 Partials 20 20 ```

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[orbeckst]

@ryankzhu would you like to review the added tutorial?

The built docs are at https://alchemlyb--379.org.readthedocs.build/en/379/tutorial.html

[orbeckst]

@xiki-tempula let's wait with merging for at least 2 days to give our JOSS reviewers a chance to look at it and ask for changes.

[ryankzhu]

Thank you for adding the tutorial section to the docs! The tutorial looks good to me.