Upgrade to mdx v2

[gabriele-ct]

• Upgrade PR: https://github.com/commercetools/commercetools-docs-kit/pull/1355

• Migration guide: https://www.gatsbyjs.com/plugins/gatsby-plugin-mdx/#migrating-from-v3-to-v4

• Upgrade PR: https://github.com/commercetools/commercetools-docs-kit/pull/1223

• Migration guide: https://mdxjs.com/migrating/v2/

Probably worth upgrading the plugins within a single PR and not taking in account existing PRs

[gabriele-ct]

• [x] Allocate couple of days to follow the migration guides, upgrade the plugins and document the most complex blockers https://github.com/commercetools/commercetools-docs-kit/issues/1403
• [ ] #1472
• [ ] commercetools/commercetools-docs#2795
• [ ] #1482

[FFawzy]

There is a risk factor because of a library that is being used for introspection of the mdx files

Nikolaus: note .. it would be okay to go back to the original implementation based on an HTML parser