Simplify documentation between setup guide and boot camp

[jaraco]

Having all the commands here is convenient, since people doing a setup don't have to follow links back and forth from this page to the GitHub bootcamp page. On the other hand, the bootcamp entries are also convenient on their own [^1].

I think the best approach would be to:

• list all upstream-related commands in a single step and code block[^2], with short inline comments (about what the commands do)
• have the same commands in the bootcamp, but with more in-depth discussions (e.g. why they are necessary)
• from here, link to the bootcamp so that people that want more details can find them there
• add a couple of comments about the duplicated commands as a remainder for people updating either page

[^1]: I regular revisit the entry about setting up a remote, since I do it for every new repo I clone, but not often enough that I remember the exact syntax [^2]: this includes steps 6-9. Step 5 can be separated.

Originally posted by @ezio-melotti in https://github.com/python/devguide/pull/943#discussion_r963142318