add xtask doctest

[tlyu]

Add a new xtask`` command doctestthat runsmdbook test. This ensures that rustdoc, when run from mdbook`, will use the toolchain override.

fixes #2251

[ehuss]

If we add this, then I think we should also update the documentation to guide users towards the preferred command/workflow. So for example:

• reference/dev-guide/src/examples.md

  Line 32 in e6b3010

  The Rust code blocks are tested in CI. You can verify that the samples pass by running [`mdbook test`].

• reference/dev-guide/src/tests.md

  Line 6 in e6b3010

  - [`mdbook test`](#inline-tests) --- Tests the inline Rust code blocks.

• reference/dev-guide/src/tests.md

  Lines 22 to 28 in e6b3010

  	## Inline tests
  	```sh
  	mdbook test
  	```
  	This command runs all tests that are inline in the Markdown. Internally, this uses [`rustdoc`](https://doc.rust-lang.org/rustdoc/) to run the tests and supports all the same features. Any code block with the `rust` language will be compiled unless it is ignored. See [Examples] for more.

View changes since this review

[tlyu]

Thanks for the review! Oops, I had some edits in progress before I saw your comments. Will incorporate them soon.

[tlyu]

I think the latest revision addresses your comments. Please let me know what you think!