💄 Make TODO chapters in Book as unclickable

[s373r]

To prevent opening empty pages this PR makes links unclickable from visual and interaction perspectives.

Please check the following preview images:

Before	After

Inspired by https://github.com/rust-lang/async-book

[yoshuawuyts]

It seems this might be causing CI to fail; if we could fix that we should be good to merge!

[s373r]

Hello all!

I've made a little investigation why we got a fail as a result of this CI job (and more interested for me why I couldn't get that locally)

Results

• "You can't have an empty link." error check added in 0.1.0 version of mdBook
• In 0.4.0 version that error check has been removed
• CI uses pinned 0.3.7 version that leads to the error

Some thoughts

• Since CI is an internal thing I cannot update the version from 0.3.7 to 0.4.X -- also, I don't know the real reasons why the version is pinned currently (it may be some needed workaround 🤔 )

Possible next steps

1. The version can be updated if we have no strong reason not to do so

OR

2. This PR can wait for version updating and I'll update it later if needed

---

@Fishrock123 & @yoshuawuyts please see ^^^ if something seems unclear feel free to ping me

Anyway I will wait for feedback :)

[s373r]

Added 40741ff that should fix CI build error

[joshtriplett]

We still want to pin a version, we should just update the pin from 0.3 to 0.4.

(Also, there's a typo: "mdbookk" should be "mdbook".)