Developer docs: integration tests and how-to-release


Linked to #82 and a comment in #61.


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> Add the tag as an “interesting checkout”

What does [this mean](https://github.com/smondet/build-docs-workflow/commit/e6da63802e07435400ab772196b32ba6646e878a#diff-7c14d634f64aeb06eba1e0e02aebfb80R284)? Is this a standard OCaml thing?