feat: add github action for markdown linting

[theguy147]

Add GitHub Action for markdown linting

Description/Motivation/Screenshots

This PR would add a GitHub Action to lint Markdown files (README.md and files in docs/). This way new contributions to GEF have to use a cleaner style for the documentation and the reviews of PR can focus more on the content rather than formatting issues and other nitpicks.

How Has This Been Tested?

Architecture	Yes/No	Comments
x86-32	✖️	Replace with ✔️ if tested
x86-64	✖️
ARM	✖️
AARCH64	✖️
MIPS	✖️
POWERPC	✖️
SPARC	✖️
RISC-V	✖️
make test	✖️

Checklist

• My PR was done against the dev branch, not master.
• My code follows the code style of this project.
• My change includes a change to the documentation, if required.
• My change adds tests as appropriate.
• I have read and agree to the CONTRIBUTING document.

[theguy147]

Added continue-on-error to markdown linting step so the files do get annotated but the checks can pass nonetheless.