feat: add markdown link checker to validate links

[asmitbm]

This adds a GitHub actions workflow which runs remark validate links which will check links in Markdown files in order to see if any are returning 404s. The logs will be available in each PR and Netlify as well.

I implemented the broken anchor links detection for both npm start and npm run build. So, even if we build this project locally, we'll get warnings there.

These are the key changes:

• Added the remark validate links for checking internal links, including checking that headers exist in linked-to files
• Added that internal link checker to the CI
• Edited a few docs which had incorrect links

Edit:
This is the log we'll get on each PR: https://github.com/oras-project/oras-www/actions/runs/5541071028/jobs/10113990367?pr=223#step:5:1

Closes: #220

[TerryHowe]

/lgtm

Thanks! This doesn't check the jsx files though I guess, but big improvement.

[asmitbm]

/lgtm

Thanks! This doesn't check the jsx files though I guess, but big improvement.

Yeah it doesn't check for jsx files as of now, I will look if there's any solution available and create a separate PR.