Remove and deprecate the introspection plugin

[FFawzy]

how to handle the deprectaiton of gatsby-transformer-mdx-introspection package Suggestion:

• tell changesets that this package changes (patch version)
• change the repository URL in package.json of the introspection plugin to https://github.com/commercetools/commercetools-docs-kit/tree/@commercetools-docs/[email protected]/packages/gatsby-transformer-mdx-introspection (last current release, alternatively use the release that this change here will product). This makes this the source URL in the package.
• change README.md with a quote at the top telling that the package is deprecated because the functionality can now easily be achieved directly through reading from the MDX v2 AST.
• merge and publish together with MDX v2
• I or nicola go to npmjs.com and change the package to deprecated (optional) new PR, removing the code. The final NPM release will point to the tag that is the last code version