proposal: Add and enable mdbook-link check by default

[jzohrab]

Checklist

• I am using the latest version of this action.
• I have read the latest README and followed the instructions.
• I have read the latest GitHub Actions official documentation and learned the basic spec and concepts.

Describe your proposal

Thank you for the helpful project!

Checking for broken links is such a common and necessary activity that adding default support for it would be super.

https://github.com/Michael-F-Bryan/mdbook-linkcheck#getting-started has a short script for getting a link checker.

Describe the solution you'd like

I think that all that would be needed would be for users to specify a step in their toml file:

[output.linkcheck]

At least on my machine, when I set up the link checker with the below script, everything runs as expected:

RELEASE=https://github.com/Michael-F-Bryan/mdbook-linkcheck/releases/download/v0.7.7/mdbook-linkcheck.x86_64-apple-darwin.zip
mkdir -p mdbook-linkcheck && cd "$_" && \
  curl -L $RELEASE -o mdbook-linkcheck.zip && \
  unzip "$_" && \
  chmod +x mdbook-linkcheck && \
  export PATH=$PWD:$PATH && \
  cd ..

Describe alternatives you've considered

Perhaps a short link in the README to the link checker.

Additional context

Thank you very much!