Add episode on branching

[davidwilby]

This draft PR is intended as a starting point for discussion on this topic as requested in #1022

I've added the episode on branches from https://github.com/carpentries-incubator/git-novice-branch-pr/ more or less verbatim albeit with name and format changes to match the new narrative. This is just added as 06b-branches.md for now to avoid complicating the diff by changing all the other file numbers but showing where it would fit into the order.

Points of discussion:

• Timing: I'm reluctant to contribute to the bloat of lessons, though I think this is a very important topic to include in this lesson.
• Should pull requests also be covered in this lesson? They are already covered with a good example in the incubator lesson linked above.
• How/should the originators of the material used from the incubator lesson be credited here?
• This branching episode uses examples of a python and a bash version of an analysis as its branches - would it be preferable to use something more aligned with the food/recipe themes instead?

Looking forward to a lively discussion on this and thanks for the opportunity to contribute!

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