Add an how-to add a new swh package

The following document demonstrates how to create a new swh package in the current swh phabricator instance and reference it so the Continuous Integration (CI) is able to build the new module.

It's problem oriented so it's more an how-to [1]. I've added it to the "tutorials" tree though as we don't have a "how to" section yet...

It might be even possible this could go to the "contribute" section but as not all steps could be done from an external contributor, I kept it out.

Anyway, feel free to propose some improvments ;)

• [1] https://documentation.divio.com/ (that's the approach we are trying to reach)

Related to #3179 (closed)

Test Plan

make -C docs is happy

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Migrated from D5968 (view on Phabricator)