extract plain markdowndown internal links

What's Changed

• Add 'unist' dependencies in package.json by @rharink in #52
• feat: extract link target from markdown link and show it in backlinks by @hikerpig in #54

New Contributors

• @rharink made their first contribution in #52

Full Changelog: https://github.com/hikerpig/gatsby-project-kb/compare/[email protected]