Fix Introduction MDX for Next.js and Typescript

[valentinpalkovic]

Resolve #20465

What I did

Changed the name of the Introduction.stories.mdx template to Introduction.mdx in Next.js Typescript projects to properly overwrite the default Introduction.mdx file.

Renamed: code/frameworks/nextjs/template/cli/ts-legacy/Introduction.stories.mdx to code/frameworks/nextjs/template/cli/ts-legacy/Introduction.mdx

How to test

Checklist

• Make sure your changes are tested (stories and/or unit, integration, or end-to-end tests)
• Make sure to add/update documentation regarding your changes
• If you are deprecating/removing a feature, make sure to update
  MIGRATION.MD

Maintainers

• If this PR should be tested against many or all sandboxes,
  make sure to add the ci:merged or ci:daily GH label to it.
• Make sure this PR contains one of the labels below.

["cleanup", "BREAKING CHANGE", "feature request", "bug", "documentation", "maintenance", "dependencies", "other"]

[yannbf]

LGTM!

[yannbf]

Let's wait for #20870 to be merged and then rebase again?

[yannbf]

@valentinpalkovic I figured out what's causing the issue.

We have a test for our event-logger (telemetry) which checks for the amount of generated stories in the sandboxes. It expects that the sandbox should contain 8 "CLI generated" stories, and 3 docs. Here's something to visualize this better:

However the resulting Storybook for nextjs/default-ts is this, which is missing the basic stories and their docs, only includes 1 docs entry which is "Introduction":