feat: add ESLint config for markdown linting with eslint-plugin-markdown

[AbhishekG-Codes]

Points to be noted:

• Some examples in the README are intentionally invalid (for illustration purposes).
• With this setup, we can start linting the "good" examples to ensure they stay clean and consistent.

Looking ahead:

• We can explore ways to skip linting the intentionally "bad" examples either by using eslint-disable comments or setting up smarter filters.

[ljharb]

Adding a new plugin is both a breaking change and a big lift - we won't be doing that.