Rollup merge of rust-lang#142636 - lolbinarycat:bootstrap-example-unsection, r=Kobzol

bootstrap.example.toml: use less contextual format

prefixing each key with its section means you don't need to scroll up 4 pages to see which section
a particular key is from.

target specific options were kept in old format since the exact section name depends on the target, so those options must now be moved to the bottom of the file.

[inspired by zulip discussion](https://rust-lang.zulipchat.com/#narrow/channel/326414-t-infra.2Fbootstrap/topic/consider.20rewriting.20bootstrap.2Eexample.2Etoml.20to.20use.20section.2Ekey/with/521734873)

r? ``@workingjubilee``

Author: jdonszelmann