Rename introduction to welcome

[goosewin]

Description

• Renamed introduction.mdx to welcome.mdx in fern/quickstart and fern/tools to align with a more welcoming nomenclature.
• Updated the corresponding page titles and slugs in the frontmatter.

Testing Steps

• Run the app locally using fern docs dev or navigate to preview deployment
• Ensure that the changed pages and code snippets work

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[cursor]

Cursor Agent can help with this pull request. Just @cursor in comments and I'll start working on changes in this branch.
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