Update and rename overview.mdx to Intro to Webb.mdx

[thomivy]

Summary of changes

This page was an overview of several things but included a description of the Anchor System. I've clarified the purpose of the page, to introduce them to Webb (not the docs, not the anchor system.)

Proposed area of change

Put an x in the boxes that apply.

• [X] Overview
• [ ] Anchor System
• [ ] dApps
• [ ] Tangle
• [ ] Protocols
• [ ] Relayers

Reference issue to close (if applicable)

Improves #70 and #97

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Code Checklist

• [x] I have tested that prove my changes are working as intended
• [x] I have added the necessary documentation (if appropriate)

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