Fix heading levels in several episodes

[jcohen02]

From the initial editorial checks carried out as part of the peer review process (see carpentries-lab/reviews#15), it's been identified that the heading levels need adjusting in several of the episodes.

In the following episodes, third level headings (###) are being used in these sections where second level headings should be used instead:

• Introduction
• Introducing the Docker Command Line
• Container Orchestration (extras)
• Creating Containers on the Cloud (extras)

In Using Docker with Github Actions (extras), there are several top level headings (#) being used, this needs to be addressed by moving all the section headings down one level.