add cmd/check-markdown/kata-check-markdown to .gitignore

[liubin]

After running ./.ci/static-checks.sh --docs there will be a file named cmd/check-markdown/kata-check-markdown remained.

[jodh-intel]

@wainersm - are you still looking at this? If not, could I possibly hijack it and give it to one of our outreach students?

[jodh-intel]

@Toreseen - if you do this, you'll need to:

$ git clone https://github.com/kata-containers/tests
$ git remote add Toreseen-github https://github.com/Toreseen/tests
$ git checkout -b update-gitignore

Then,

• Browse to https://github.com/kata-containers/tests and click "fork".
• Run the command @liubin has specified above.
• Check that the output of git status shows cmd/check-markdown/kata-check-markdown.
• Modify the top-level .gitignore to add cmd/check-markdown/kata-check-markdown.
• Re-run git status and ensure that cmd/check-markdown/kata-check-markdown is no longer shown.

Then,

$ git commit -as
$ git push -f Toreseen-github