Auto merge of #8083 - t-nelis:mdbook-v0.3.7, r=ehuss

Upgrade to mdBook v0.3.7

This bumps the requirement from Rust v1.34.0 to v1.35.0 for building docs. AFAICT CI is using nightlies so that should be fine, but I thought I'd mention it in case someone thinks this impacts contributors in any way.

Other than that, there are a few changes that might impact some users in a visible way, like automatic dark theme support for those who picked that perference in their browser, possible color changes to the scrollbar and to the font size, change in the spacing in the sidebar entries, and many more changes and fixes that won't be too immediately impactful but very good all around.

I checked changes from transitive dependency bumps as well, AFAICT there is nothing that *should* impact the final rendering.

**tl;dr:** Nothing will explode. Probably.

For completeness, my raw notes of outtakes as I was reviewing the change logs:

```
[cosmetic]
v0.3.2 https://github.com/rust-lang/mdBook/blob/master/CHANGELOG.md#mdbook-032
Added automatic dark-theme detection based on the CSS prefers-color-scheme feature. This may be enabled by setting output.html.preferred-dark-theme to your preferred dark theme. #1037
rust-lang/mdBook#1037

v0.3.3 https://github.com/rust-lang/mdBook/blob/master/CHANGELOG.md#mdbook-033
Improvements to the automatic dark theme selection. #1069
rust-lang/mdBook#1069

v0.3.7 https://github.com/rust-lang/mdBook/blob/master/CHANGELOG.md#mdbook-037
Fixed theme selector focus. #1170
rust-lang/mdBook#1170

* https://developer.mozilla.org/en-US/docs/Web/CSS/@media/prefers-color-scheme
* users who picked the dark color scheme in their browser will see the cargo doc in dark.

[cosmetic]
v0.3.2 https://github.com/rust-lang/mdBook/blob/master/CHANGELOG.md#mdbook-032
Use standard scrollbar-color CSS along with webkit extension #816
rust-lang/mdBook#816

* https://developer.mozilla.org/en-US/docs/Web/CSS/scrollbar-color
* scroll bar color might change i guess.

[helpful]
v0.3.2 https://github.com/rust-lang/mdBook/blob/master/CHANGELOG.md#mdbook-032
Updated highlight.js for syntax highlighting updates (primarily to add async/await to Rust highlighting). #1041
rust-lang/mdBook#1041

* not sure cargo doc has many code examples with async/await, but there we go.

[warning]
v0.3.2 https://github.com/rust-lang/mdBook/blob/master/CHANGELOG.md#mdbook-032
Raised minimum supported rust version to 1.35. #1003
rust-lang/mdBook#1003

* from 1.34.0.

[cosmetic]
v0.3.4 https://github.com/rust-lang/mdBook/blob/master/CHANGELOG.md#mdbook-034
Switch to relative rem font sizes from px. #894
rust-lang/mdBook#894

* will impact some displays, but px is already an abstract thing so maybe not that big of an impact.

[warning]
v0.3.5 https://github.com/rust-lang/mdBook/blob/master/CHANGELOG.md#mdbook-035
Updated pulldown-cmark to 0.6.1, fixing several issues. #1021
rust-lang/mdBook#1021

* from 0.5, breaking changes.
* parsing only -- the team had to do multiple changes but nothing seems like it would impact final rendering

[cosmetic]
v0.3.6 https://github.com/rust-lang/mdBook/blob/master/CHANGELOG.md#mdbook-036
Adjusted spacing of sidebar entries. #1137
rust-lang/mdBook#1137

[warning]
v0.3.6 https://github.com/rust-lang/mdBook/blob/master/CHANGELOG.md#mdbook-036
Handlebars updated to 3.0. #1130
rust-lang/mdBook#1130

* from 2.0
* https://github.com/sunng87/handlebars-rust/blob/master/CHANGELOG.md
* strictly maintenance and perf AFAICS, no changes to final rendering.

[cosmetic]
v0.3.6 https://github.com/rust-lang/mdBook/blob/master/CHANGELOG.md#mdbook-036
Adjusted the menu bar animation to not immediately obscure the top content. #989
rust-lang/mdBook#989

* personal fave.

[cosmetic]
v0.3.7 https://github.com/rust-lang/mdBook/blob/master/CHANGELOG.md#mdbook-037
Code spans in headers are no longer highlighted as code. #1162

* users will see some headers change, probably.

[fixes]
+ ~13 fixes impacting rendering in less immediately visible ways.
rest should have no impact on end-user experience.
```

azure-pipelines.yml

@@ -98,7 +98,7 @@ jobs:
     - bash: |
         set -e
         mkdir mdbook
-        curl -Lf https://github.com/rust-lang-nursery/mdBook/releases/download/v0.3.1/mdbook-v0.3.1-x86_64-unknown-linux-gnu.tar.gz | tar -xz --directory=./mdbook
+        curl -Lf https://github.com/rust-lang/mdBook/releases/download/v0.3.7/mdbook-v0.3.7-x86_64-unknown-linux-gnu.tar.gz | tar -xz --directory=./mdbook
         echo "##vso[task.prependpath]`pwd`/mdbook"
       displayName: "Install mdbook"
     - bash: cargo doc --no-deps