Add callouts explaining branches

[alex-ball]

These changes provide the explanation of branches that is promised but not delivered in the lesson.

In order to keep the cognitive load light, I've split the explanation into two parts:

• In episode 2, following the use of git status, we explain branches as labelled sequences of commits. This is all we need for episodes 2-4.
• In episode 5, following the first pull request challenge, we explain what just happened in terms of branching and merging.

Each of these is illustrated with a diagram. The first one is there primarily to make the second one seem more familiar.

• In common with how the Git Book does it, the arrows between commits point backwards in time. I've used triangular arrow heads to evoke how UML shows class inheritance: each commit "inherits" the last one then changes it a bit. The callout in episode 2 says commits point back to their parents, to explain this.
• I used a dashed arrow for the merged-parent relationship to emphasise that merging isn't a symmetrical operation.
• I have used colours from the stylesheet so they tone with the rest of the site; these choices seem to work well in both light and dark mode, normally and when viewed in greyscale.
• The alt texts describe the diagrams structurally rather than captioning them, in an attempt to improve accessibility.

Addresses #54.

[github-actions]

Thank you!

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Rendered Changes

🔍 Inspect the changes: https://github.com/LibraryCarpentry/lc-git/compare/md-outputs..md-outputs-PR-190

The following changes were observed in the rendered markdown documents:

 02-getting-started.md      |  15 +++-
 05-github-pages.md         |  14 +++
 fig/git-branch.svg (new)   | 129 +++++++++++++++++++++++++++
 fig/git-branches.svg (new) | 214 +++++++++++++++++++++++++++++++++++++++++++++
 md5sum.txt                 |   4 +-
 5 files changed, 373 insertions(+), 3 deletions(-)

What does this mean?

If you have source files that require output and figures to be generated (e.g. R Markdown), then it is important to make sure the generated figures and output are reproducible.

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⏱️ Updated at 2025-08-29 12:05:05 +0000

[srerickson]

This is really great. I was considering whether to expand the initial callout to explain the the idea of an active branch, but I think simpler is better, especially since we don't talk about "HEAD" in this lesson. The SVGs look great in both light/dark modes.