Add branch to index.

[andersoncedu]

Issue: There was no reference to branch in index even though it's an important concept in Open Source.

Fix: Change sec_git_branching.ptx to add branch as an index in reference to first paragraph of section.

Test: To test these changes, build the pretext book, view the changes and ensure that the reference to branch in the index references the first paragraph of the git branching section.

Reviewed by Isaac Gray.

Fix #258

[pearcej]

@andersoncedu Can you update your branch so you are not removing an alt tag?

[andersoncedu]

I updated the branch to revert my omission of the description.

[andersoncedu]

I updated my fork to match the original repo and made all requested changes.

[pearcej]

Looks good now. Thanks!