Add documentation of tox and GitHub actions workflow to developer's manual

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CC:  @dimpase @jhpalmieri @videlec @tscrim @orlitzky @kiwifb @slel

Component: **documentation**

Author: **Matthias Koeppe**

Branch/Commit: **[`99aca39`](https://github.com/sagemath/sagetrac-mirror/commit/99aca39dea0226372a3f12aed3a13301a3380ff4)**

Reviewer: **Dima Pasechnik**

_Issue created by migration from https://trac.sagemath.org/ticket/29401_

