Docs(merge_strategies): Clarfiy rebase and ff merge similarity

[devtayls]

Why:

Closes 1387

What's being changed:

Content Changes

Pull requests with rebased commits are merged using the fast-forward option.

to:

In that way, the rebase and merge behavior resembles a fast-forward merge by maintaining a linear project history. However, rebasing achieves this by re-writing the commit history on the base branch with new commits.

This is because as @bk2204, @spikyjt explained in the issue, rebasing looks like ff-merge, but in actuality is a destructive method which changes the project history.

Refactor

Move paragraph on enabling Rebase and Merge behind paragraph explaining how rebasing works on Github. This flows better as: 'What is rebase and merge?' -> 'How do I enable Rebase and Merge?' Than the previous version which was: 'What is rebase and merge?' -> 'How do I enable Rebase and Merge' -> 'How github rebase differs from git rebase'

Check off the following:

• I have reviewed my changes in staging (look for the latest deployment event in your pull request's timeline, then click View deployment).
• For content changes, I have completed the self-review checklist.

Writer impact (This section is for GitHub staff members only):

• This pull request impacts the contribution experience
  • I have added the 'writer impact' label
  • I have added a description and/or a video demo of the changes below (e.g. a "before and after video")

[welcome]

Thanks for opening this pull request! A GitHub docs team member should be by to give feedback soon. In the meantime, please check out the contributing guidelines.

[ramyaparimi]

@devtayls
Thanks so much for opening a PR! I'll get this triaged for review ⚡

[lecoursen]

Thanks @devtayls, this looks great! I'm applying a suggestion with a few small style tweaks and will merge after CI finishes! ⚡

[lecoursen]

The link check errors are unrelated to this PR.

[github-actions]

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