Update some codespaces screenshots/wording

[pcai]

• creating a repo from a template no longer shows a template selector; update screenshot to reduce confusion. also, they removed "Include all branches" as an option since that is part of the template selector ui
• we removed the README from the template repo, because if we include one, rails new . will stop and prompt users to overwrite (because rails new wants to create a README as well). This is confusing to students, so we removed it, instead of writing "wait a long time for nonsense to spit out, but also, if it stops in the middle and asks you a question you dont understand, you should enter "y" and hit enter". Updated the screenshot of the default codespaces landing page to reflect that there is indeed no README when VSC initially opens.
• update wording to make it more clear that a 2-3 minute wait while its building the docker container for the codespace is expected.