Link to skill tree and fix some nodes

[RalfJung]

r? @oli-obk
Fixes #51

[RalfJung]

offset_of has very little to do with offset/wrapping_offset. If we want the latter in the skill tree, they should be a separate node. EDIT: done that.

[oli-obk]

this used to contain more RFCs, but they have been moved to tracking issues since. I was thinking that we should somehow highlight that some node is an RFC and not something already accepted (though... considering the random collection of issues I opened that doesn't have a merged RFC, I guess we can just treat them the same)

[RalfJung]

Hm, https://rust-lang.github.io/const-eval/ still contains that "rfc" node?

[RalfJung]

CI said

Warning: The mdbook-skill-tree preprocessor was built against version 0.3.7 of mdbook, but we're being called from version 0.4.1
thread 'main' panicked at 'encountered error missing field `items` for key `group` at line 279 column 1', mdbook-skill-tree/src/preprocessor.rs:85:31
note: run with `RUST_BACKTRACE=1` environment variable to display a backtrace
2020-08-10 13:22:09 [ERROR] (mdbook::utils): Error: The preprocessor exited unsuccessfully
##[error]Process completed with exit code 101.

[RalfJung]

I did a direct push to hopefully fix this.