Auto merge of #42612 - est31:master, r=nagisa

Autogenerate stubs and SUMMARY.md in the unstable book

Removes a speed bump in compiler development by autogenerating stubs for features in the unstable book. See #42454 for discussion.

The PR contains three commits, separated in order to make review easy:

* The first commit converts the tidy tool from a binary crate to a crate that contains both a library and a binary. In the second commit, we'll use the tidy library
* The second and main commit introduces autogeneration of SUMMARY.md and feature stub files
* The third commit turns off the tidy lint that checks for features without a stub, and removes the stub files. A separate commit due to the large number of files touched

Members of the doc team who wish to document some features can either do this (where `$rustsrc` is the root of the rust repo git checkout):

1. cd to `$rustsrc/src/tools/unstable-book-gen` and then do `cargo run $rustsrc/src $rustsrc/src/doc/unstable-book` to put the stubs into the unstable book
2. cd to `$rustsrc` and run `git ls-files --others --exclude-standard` to list the newly added stubs
3. choose a file to edit, then `git add` it and `git commit`
4. afterwards, remove all changes by the tool by doing `git --reset hard` and `git clean -f`

Or they can do this:

1. remove the comment marker in `src/tools/tidy/src/unstable_book.rs` line 122
2. run `./x.py test src/tools/tidy` to list the unstable features which only have stubs
3. revert the change in 1
3. document one of the chosen unstable features

The changes done by this PR also allow for further development:

* tidy obtains information about tracking issues. We can now forbid differing tracking issues between differing `#![unstable]` annotations. I haven't done this but plan to in a future PR
* we now have a general framework for generating stuff for the unstable book at build time. Further changes can autogenerate a list of the API a given library feature exposes.

The old way to simply click through the documentation after it has been uploaded to rust-lang.org works as well.

r? @nagisa

Fixes #42454

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