Add details of each setting in the toolchain file

[nicholasbishop]

The format of the new section is modeled after the cargo docs, for example: https://doc.rust-lang.org/cargo/reference/profiles.html#profile-settings

This is mostly a reorganization of the existing content to make it easier to scan for relevant details, but a few new details have also been added:

1. For channel, the format is now explained in the same format as in concepts/toolchains.md, but with the more limited form that rust-toolchain.toml accepts. This is based on the text that was here previously; I haven't verified if the code is actually this strict.
2. The profile section mentions that the default profile is not necessarily the profile named default, which is subtle detail that can be confusing.
3. The components section notes that the list is additive with the profile's list of components.
4. The targets field is now documented, previously it wasn't mentioned other than to say it's ignored with path.

[0xPoe]

You can check out this rendered page.

[nicholasbishop]

Friendly ping, would love to get this merged :)

[0xPoe]

I am going to merge this PR. If we find some grammar issues we can fix them later. Thanks! Sorry for the long wait.