Update remaining images to recipes and main

[ndporter]

How could the content be improved?

Many of the images in /episodes/fig/ still reference the master branch and/or planets motivating example. Here is a list of the images that appear to need updating and how:

• github-change-repo-string.png: change to recipes
• github-create-repo-02.png: change to recipes
• github-create-repo-03.png: change to recipes
• github-find-repo-string.png: change to recipes
• motivatingexample.png: remove from repo (it only applies to planets)
• RStudio_*.png: update branch name to main and screenshots to reflect recipes, not planets (may need updated version of RStudio as well, though I didn't check)

To reduce cognitive load, it may also be worth redoing all the ones that show Github usernames to reflect the same user - but that's a minor concern compared to getting everything matched to the current instructions.

(submitted by recommendation of @sarahover)

[martinosorb]

Most of these have been fixed by #1068. However, the RStudio ones have not.

[ndporter]

Thanks @martinosorb. I may be able to fix the RStudio ones in February if nobody's managed by then and someone @'s me to remind me.

[martinosorb]

@ndporter thank you, this has been done by Erin. Do let us know if you still see any problems.