Add support for union types in raml docs for api types

[timonrey]

Description of changes This PR refactores the pre processing of resource parameters and api props. It also adds support for union types for api types.

Link original ticket closes #1483

Screenshot of changes (if applicable) You can find an example here: https://docskit-tt-refactor-type-rendering-for-types-and-endpoints.commercetools.vercel.app/api-docs-smoke-test/types/general#unionreferencetesttype

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🦋 Changeset detected

Latest commit: d8099ae3019b5cffc28a24520450832ece44f091

The changes in this PR will be included in the next version bump.

This PR includes changesets to release 3 packages

| Name | Type | | ------------------------------------------- | ----- | | @commercetools-docs/gatsby-transformer-raml | Patch | | @commercetools-docs/gatsby-theme-api-docs | Patch | | @commercetools-website/api-docs-smoke-test | Patch |

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🚚 Build Process Details

Preview URLs:

• https://docskit-tt-refactor-type-rendering-for-types-and-endpoints.commercetools.vercel.app (permalink to the last build of this branch)
• https://docskit-sha-91a4ca1351abf0659135e4efe6eefcb5584b5fe3.commercetools.vercel.app (this commit)