AST-1521: add direct link & RTD use poetry env

It looks like the readthedocs has been failing to build properly since the dependencies required for building the documentation has been moved to an optional group in the poetry toml file. It is possible that we missed that because it did build succesfully before this move.

Here is some extra background on why Chiara and I chose for this solution. The page with an overview of builds is for instance here. We noticed that since my changed RTD source files, we got an error that indicates the dependencies didn't got installed properly (see this build). Once we added the changes to the readthedocs.yaml file as in this MR, this was solved. See this build. We had to specifically instruct it to only build the doc-dependencies (--only docs), otherwise lsmtool had issues installing properly. See this build.