Add GitHub workflow to check links in markdown files

[mingxin-zheng]

No description provided.

[mingxin-zheng]

Hi @finalelement , this new CI revealed that the following links are invalid:

FILE: ./monai_vila2d/data_prepare/README.md
  [✖] https://github.com/UCSD-AI4H/PathVQA
  [✓] https://github.com/abachaa/VQA-Med-2019
  [✖] https://github.com/SLAKE-SLAKE/SLAKE
  [✓] https://physionet.org/content/medical-diff-vqa/1.0.0

I saw the file is recently touched by you. Would you mind letting me know which links should be replaced by? Thanks!

[mingxin-zheng]

According to offline chat with @finalelement , he will push a fix in this week.
Shall we just merge this, leaving the check optional? @holgerroth

[finalelement]

Links updated @mingxin-zheng

[mingxin-zheng]

Thanks @finalelement , after your PR being merged, this check will all passed, as shown here:
#22

[holgerroth]

This is useful!