Fix multiple hyperlinks in README

[cxheng315]

Thank you for contributing to Spellbook 🪄

Please open the PR in draft and mark as ready when you want to request a review.

Description:

1. Updated the "Submitting a PR" link to direct to the correct section (#submitting-a-pr).
2. Updated the "Setting up your dev environment" link to direct to the correct section (#setting-up-your-local-dev-environment).
3. Updated the "Using dbt to write spells" link to direct to the correct section (#how-to-use-dbt-to-create-spells).
4. Updated the "Issues" link to point to the correct GitHub Issues page (https://github.com/duneanalytics/spellbook/issues).
5. Redirect walkthrough to better place(#introduction).

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quick links for more information:

• README.md
• spellbook docs
• CONTRIBUTING.md

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[cxheng315]

I have read the CLA Document and I hereby sign the CLA