Add markdownlint validation and eslint for website

[vdusek]

Summary

Add markdownlint validation for documentation, and consider enabling eslint (oxlint + oxfmt) for the website project.

Tasks

• Add markdownlint and markdownlint-cli to website/package.json
• Fix documentation content to pass markdownlint validation
• Add lint:md and lint:md:fix scripts to package.json
• Add markdown linting to CI as a docs-check step
• Consider enabling eslint (oxlint + oxfmt) for website/ project

Context

This will catch common markdown issues (formatting, broken links, etc.) automatically in CI, similar to Python code validation.

Based on discussion: apify/apify-sdk-python#747 (comment)