Add an how-to create a new swh package

From new swh repository to a documented package step-by-step:

• new repository in forge (tags, ...), see https://wiki.softwareheritage.org/wiki/Phabricator_callsign_naming_convention for the callsign
• clone out of py-template
• reference in .mrconfig
• add ci jobs (unit test, debian, ...)
• (optionally) prepare debian tree...
• reference the module swh-docs

Draft already exists [1]

The next step is to actually create a page in swh-docs about it.

• [1] https://hedgedoc.softwareheritage.org/CaqENEY3RJi19idS8MggWw?view#documentation-updates

---

Migrated from T3179 (view on Phabricator)